programming php

Php for the web pdf free download

Description

With PHP for the World Wide Web, Fourth Edition: Visual QuickStart Guide, readers can start from the beginning to get a tour of the programming language, or look up specific tasks to learn just what they need to know. This task-based visual reference guide uses step-by-step instructions and plenty of screenshots to teach beginning and intermediate users this popular open-source scripting language. Leading technology author Larry Ullman guides readers through the latest developments including use and awareness of HTML5 with PHP. Other addressed changes include removal of outdated functions and more efficient ways to tackle common needs. Both beginning users, who want a thorough introduction to the technology, and more intermediate users, who are looking for a convenient reference, will find what they need here–in straightforward language and through readily accessible examples.

Open the PDF directly: View PDF link";

One workaround is to

escape

the quota-

tion marks within the HTML by preceding

them with a backslash [

print "link";

By escaping each quotation mark within

the

statement, you tell PHP to print

the mark itself instead of treating the quo-

tation mark as either the beginning or the

end of the string to be printed.

To send HTML to the browser:

1. Open the

hello1.php

script [Script 1.3]

in your text editor or IDE, if it is not

already open.

2. Within the HTML head, declare a CSS

class [Script 1.4]:

.bold {

font-weight: bolder;

}

continues on next page

A Attempting to print double quotation marks will

create errors, because they conflict with the

statement’s primary double quotation marks.

Script 1.4 Using

, you can send HTML

tags along with text to the browser, where the

formatting will be applied.

5 Hello, World!

7 .bold {

8 font-weight: bolder;

9 }

The following was created by PHP:

ptg18144795

22 Chapter 1

This CSS code declares a class named

bold

, which will be used to add empha-

sis to text. This is obviously a fairly trivial

use of CSS, but by declaring this as a

class, it can easily be updated, perhaps

to change the color of the text or the

size, along with its weight.

3. Edit the

Hello, world!

message by

adding HTML tags, making it read as

follows:

print "

➝

Hello, world!";

To make the PHP-generated part of

the message stand out, CSS styling will

bold the greeting. For this to work, you

must escape the quotation marks within

the span tag so they don’t conflict with

the

statement’s quotation mark.

4. Save the script as

hello2.php

, place

it on your PHP-enabled server, and run

the page in your browser B.

B The new version of the

Hello, world!

page, with

a little more decoration and appeal.

Using White Space

When programming in PHP, white space is generally [but not universally] ignored. Any blank line

[just one or several in a row] in PHP code is irrelevant to the end result. Likewise, tabs and spaces

are normally inconsequential to PHP. And because PHP code is not visible in the browser [unless

there’s a problem with the server], white space in your PHP files has no impact on what the end

user sees.

The spacing of HTML code shows up in the HTML source of a web page but has only a minimal

effect on what’s viewed in the browser. For example, all of a page’s HTML source code could be

placed on one line without changing what the end user sees. If you had to hunt for a problem in

the HTML source, however, you would not like the long, single line of HTML.

You can affect the spacing of dynamically generated HTML code by printing it in PHP over multiple

lines, or by using the newline character [

] within double quotation marks:

print "Line 1\nLine 2";

Again, use of the newline character affects the

HTML source code

of the web page, not what the

end user sees rendered in the browser.

To adjust the spacing in the rendered web page, you’ll use CSS, plus paragraph, div, and break

tags, among others.

ptg18144795

Getting Started with PHP 23

Understanding the role of quotation

marks and how to escape problematic charac-

ters is crucial to programming with PHP. These

topics will be covered in more detail in the

next two chapters.

The HTML you send to the web browser

from PHP doesn’t need to be this simple. You

can create tables, JavaScript, and much, much

more.

Remember that any HTML outside the

PHP tags will automatically go to the browser.

Within the PHP tags,

statements are

used to send HTML to the web browser.

C The resulting HTML source code of

hello2.php

5. View the HTML page source to see the

code that was sent to the browser C.

How you do this depends on the

browser: Select View > Developer >

View Source in Chrome, View > Page

Source in Firefox, or View > Source in

Internet Explorer.

This is a step you’ll want to be in the

habit of taking, particularly when prob-

lems occur. Remember that PHP is

primarily used to generate HTML, sent

to and interpreted by the browser.

Often, confirming what was sent to the

browser [by viewing the source] will help

explain the problem you’re seeing in the

browser’s interpretation [or visible result].

ptg18144795

24 Chapter 1

Adding Comments

to Scripts

Comments are integral to programming,

not because they do anything but because

they help you remember why

you

did

something. The computer ignores these

comments when it processes the script.

Furthermore, PHP comments are never

sent to the browser, remaining your secret.

PHP supports three ways of adding

comments. You can create a single-line

comment by putting either

at the

beginning of the line you want ignored:

// This is a comment.

You can also use

to begin a com-

ment at the end of a PHP line, like so:

print "Hello"; // Just a greeting.

Although it’s largely a stylistic issue,

much more commonly used in PHP than

You can create a multiline comment using

to begin the comment and

to con-

clude it:

/* This is a

multi-line comment. */

Some programmers prefer this comment

style because it contains both open and

closing “tags,” providing demarcation for

where the comment begins and ends.

ptg18144795

Getting Started with PHP 25

To add comments to a script:

1. Open the

hello2.php

created earlier

[Script 1.4] in your text editor or IDE.

2. After the initial PHP tag, add some

comments to your script [Script 1.5]:

* Filename: hello3.php

* Book reference: Script 1.5

* Created by: Larry Ullman

This is just a sample of the kind of

comments you can write. You should

document what the script does, what

information it relies on, who created it,

when, and so forth. Stylistically, such

comments are often placed at the top

of a script [as the first thing within the

PHP section, that is], using formatting

like this. The extra asterisks aren’t

required; they just draw attention to

the comments.

3. On line 21, in front of the

state-

ment, type

By preceding the

statement with

two slashes, you ensure that the func-

tion call is “commented out,” meaning it

will never be executed.

4. After the closing PHP tag [on line 23],

add an HTML comment:

This line of code will help you distin-

guish among the different comment

types and where they appear. This com-

ment will appear only within the HTML

source code.

continues on next page

Script 1.5 PHP and HTML comments are added

to the script to document it and to render a line of

PHP code inert.

5 Hello, World!

7 .bold {

8 font-weight: bolder;

9 }

The following was created by PHP:

ptg18144795

26 Chapter 1

5. Save the script as

hello3.php

, place

it on your PHP-enabled server, and run

the page in your web browser A.

6. View the source of the page to see the

HTML comment B.

You can comment out just one line of

code or several using the

and

method.

With

, you can negate only one line at

a time.

Different programmers prefer to com-

ment code in different ways. The important

thing is to find a system that works for you

and stick to it.

Note that you cannot use HTML com-

ment characters [

] within PHP to

comment out code. You could have PHP print

those tags to the browser, but in that case

you’d create a comment that appeared in the

HTML source code on the client’s computer

[but not in the browser window]. PHP comments

never make it as far as a user’s computer.

Despite my strong belief that you can’t

over-comment your scripts, the scripts in this

book aren’t as documented as they should

be, in order to save space. But the book will

document each script’s name and number, for

cross-reference purposes.

When you change a script’s code, be

certain to update its comments as well. It’s

quite confusing to see a comment that suggests

a script or a line of code does something other

than what it actually does.

A With the

statement commented out,

the page looks just as it would if the

call

weren’t there.

B HTML comments don’t appear in the web

browser but are in the HTML source. PHP

comments remain in the PHP script on the server,

not visible inside the HTML source.

ptg18144795

Getting Started with PHP 27

Basic Debugging Steps

Debugging is by no means a simple con-

cept to grasp, and unfortunately, it’s one

that is only truly mastered by doing. The

next 50 pages could be dedicated to the

subject and you’d still merely pick up a

fraction of the debugging skills that you’ll

eventually acquire and need.

The reason I introduce debugging in this

harrowing way is that it’s important not to

enter into programming with delusions.

Sometimes code won’t work as expected,

you’ll inevitably create careless errors, and

some days you’ll want to pull your hair out,

even when using a comparatively user-

friendly language such as PHP. In short,

prepare to be perplexed and frustrated

at times

. I’ve been coding in PHP since

1999, and occasionally I still get stuck in

the programming muck. But debugging is

a very important skill to have, and one that

you will eventually pick up out of necessity

and experience. As you begin your PHP

programming adventure, I offer the follow-

ing basic but concrete debugging tips.

ptg18144795

28 Chapter 1

To debug a PHP script:

Make sure you’re always running PHP

scripts through a URL!

This is perhaps the most common

beginner’s mistake. PHP code must be

run through the web server applica-

tion, which means it must be requested

through //

something

. When you

see actual PHP code instead of the

result of that code’s execution, most

likely you’re not running the PHP script

through a URL.

Know what version of PHP you’re

running.

Some problems arise from the version

of PHP in use. Before you ever use

any PHP-enabled server, run the

phpinfo.php

file [Script 1.2] to confirm

the version of PHP in use.

Make sure

display_errors

is on.

This is a basic PHP configuration set-

ting [discussed in Appendix A]. You

can confirm this setting by executing

the

phpinfo[]

function [just use your

browser to search for

display_errors

in the resulting page]. For security

reasons, PHP may not be set to display

the errors that occur. If that’s the case,

you’ll end up seeing blank pages when

problems occur. To debug most prob-

lems, you’ll need to see the errors, so

turn this setting on while you’re learn-

ing. You’ll find instructions for doing so

in Appendix A and Chapter 3, “HTML

Forms and PHP.”

Check the HTML source code.

Sometimes the problem is hidden in

the HTML source of the page. In fact,

sometimes the PHP error message can

be hidden there!

Trust the error message.

Another very common beginner’s mis-

take is to not fully read or trust the error

that PHP reports. Although an error

message can often be cryptic and may

seem meaningless, it can’t be ignored.

At the very least, PHP is normally cor-

rect as to the line on which the problem

can be found. And if you need to relay

that error message to someone else

[like when you’re asking me for help],

do include the entire error message!

Take a break!

So many of the programming problems

I’ve encountered over the years, and

the vast majority of the toughest ones,

have been solved by stepping away

from my computer for a while. It’s easy

to become frustrated and confused,

and in such situations, any further steps

you take are likely to make matters

only worse.

These are just some general debugging

techniques, specifically tailored to the begin-

ning PHP programmer. They should suffice

for now, because the examples in this book

are relatively simple. More complex coding

requires more advanced debugging tech-

niques, so my PHP and MySQL for Dynamic

Web Sites: Visual QuickPro Guide, Fourth

Edition [Peachpit Press, 2012] dedicates a

whole chapter to this subject.

ptg18144795

Getting Started with PHP 29

Review and Pursue

Each chapter in this book ends with a

“Review and Pursue” section. In these

sections you’ll find:

Questions regarding the material just

covered

Prompts for ways to expand your

knowledge and experience on your own

If you have any problems with these

sections, in either answering the questions

or pursuing your own endeavors, turn

to the book’s supporting forum

[www.LarryUllman.com/forums/].

Review

What is HTML? What is the current

version of HTML?

What encoding is your text editor or IDE

set to use? Does that match the encod-

ing specified in your generated HTML

pages? Why does the encoding matter?

What is CSS and what is it used for?

What file extension should PHP scripts

have for your particular server?

What is meant by “web root directory”?

What is the web root directory for

your server?

How do you test PHP scripts? What

happens when PHP scripts are not

run through a URL?

Name two ways comments can be

added to PHP code. Identify some

reasons to use comments.

ptg18144795

30 Chapter 1

Pursue

If you have access to more than one

server, confirm what version of PHP is

running on another server.

Create a static HTML page that displays

some information. Then replace some of

the static content with content created

by PHP.

Create a template to use for your own

work. The template should contain the

HTML shell, the opening and closing

PHP tags, and some basic comments.

Confirm, using the

phpinfo[]

function,

that

display_errors

is enabled on

your server. If it’s not, change your

server’s configuration to enable it [see

Chapter 3 and Appendix A].

In subsequent chapters, occasionally

check the PHP manual’s page when a

new function is mentioned in the book.

ptg18144795

The previous chapter covered how to use

PHP to send simple text and HTML to a

web browser—in other words, something

for which you don’t need PHP at all! Don’t

worry, though; this book will teach you how

to use

in conjunction with other PHP

features to do great and useful things with

your website.

To make the leap from creating simple,

static pages to dynamic web applications

and interactive websites, you need vari-

ables. Understanding what variables are,

the types of variables that a language sup-

ports, and how to use them is critical.

This chapter introduces the fundamentals

of variables in PHP, and later chapters

cover the different types in greater detail.

If you’ve never dealt with variables before,

this chapter will be a good introduction. If

you’re familiar with the concept, then you

should be able to work through this chapter

with ease.

Variables

In This Chapter

What Are Variables? 32

Variable Syntax 36

Types of Variables 38

Variable Values 41

Understanding Quotation Marks 44

Review and Pursue 48

ptg18144795

32 Chapter 2

What Are Variables?

variable

is a container for data. Once

data has been stored in a variable [or,

stated more commonly, once a variable

has been assigned a value], that data can

be altered, printed to the browser, saved to

a database, emailed, and so forth.

Variables in PHP are, by their nature,

flexible: You can put data into a variable,

retrieve that data from it [without affecting

the value of the variable], put new data in

it, and continue this cycle as many times

as necessary. But variables in PHP are

largely temporary:

Most only exist

—that is,

they only have a value—

for the duration

of the script’s execution on the server

Once the execution of the script completes

[often when the final closing PHP tag is

encountered], those variables cease to

exist. Furthermore, after users click a link

or submit a form, they are taken to a new

page that may have an entirely separate

set of variables.

Before getting too deep into the discus-

sion of variables, let’s write a quick script

that reveals some of PHP’s

predefined

variables. These are variables that PHP

automatically creates when a script runs.

Over the course of the book, you’ll be

introduced to many different predefined

variables. This particular example looks

at the predefined

$_SERVER

variable. It

contains lots of information about the com-

puter on which PHP is running.

The

print_r[]

function offers an easy way

to display any variable’s value:

print_r[$variable_name];

Just provide the name of the variable

you’d like to inspect as a single argument

to the

print_r[]

function. [You’ll learn

more about a variable’s syntax throughout

this chapter.]

ptg18144795

Variables 33

To print PHP’s predefined variables:

1. Create a new PHP script in your

text editor or IDE, to be named

predefined.php

[Script 2.1].

2. Create the initial HTML tags:

Predefined Variables

➝

This code repeats the HTML template

created in the preceding chapter. Within

the body of the page, the

tags

are being used to make the generated

PHP information more legible. Without

using the


 tags, the 
print_r[]
function’s output would be difficult to 
read in a browser.
3. Add
the PHP code:
The PHP code contains just one func-
tion call. The function should be pro-
vided with the name of a variable.
In this example, the variable is 
$_SERVER
, which is special in PHP. 
$_SERVER
 stores all sorts of data about 

  
the server: its name and operating 
system, the name of the current
user, 
information about the web server appli-
cation [Apache, Nginx, IIS, and so on], 
and more. It also reflects the PHP script 
being executed: its name, where it’s 
stored on the server, and so forth.
continues on next page 
Script 2.1 This script uses the 
print_r[]
 function 
to show the values stored in the 
$_SERVER
predefined variable.
1 
2 
3 
4 
5  Predefined Variables
6 
7 
8 
9 
15 
16 
17 
ptg18144795
34 Chapter 2
Note that you must type 
$_SERVER
exactly as it is here, in all upper- 
case letters.
4. Complete the HTML page:

5. Save the file as

predefined.php

upload it to your server [or save it to the

appropriate directory on your com-

puter], and test it in your browser A.

Once again, remember that you must

run all PHP scripts through a URL [that

is, //

something

6. If possible, transfer the file to another

computer or server running PHP and

execute the script in your browser

again B.

Printing out the value of any variable

as you’ve done here is one of the greatest

debugging tools. Scripts often don’t work

as you expect them to because one or more

variables do not have the values you assume

they should, so confirming their actual values

is extremely helpful.

If you don’t use the HTML

tags, the result will be like the jumble of infor-

mation in C.

A The

$_SERVER

variable, as printed out by this script, is a master list

of values pertaining to the server and the PHP script.

ptg18144795

Variables 35

B With the

predefined.php

page, different servers will generate different

results [compare with A].

C With large, complex variables such as

$_SERVER

, not using the HTML

preformatting tags with

print_r[]

creates an incomprehensible mess

[compare to A B].

ptg18144795

36 Chapter 2

Variable Syntax

Now that you’ve had a quick dip in the vari-

able pool, it’s time to swim a bit deeper. In

the preceding example, the script printed

out the value of PHP’s predefined

$_SERVER

variable. You can also create your own

variables, once you understand the proper

syntax. To create appropriate variable

names, you must follow these rules:

All variable names must be preceded

by a dollar sign [

Following the dollar sign, the variable

name must begin with either a letter

[A–Z, a–z] or an underscore [

]. A num-

ber cannot immediately follow the

dollar sign.

The rest of the variable name can con-

tain any combination of letters, under-

scores, and numbers.

You may not use spaces within the

name of a variable. [Instead, the under-

score is commonly used to separate

words.]

Each variable must have a unique

name.

Variable names are

case-sensitive

Consequently,

$variable

and

$Variable

are two different constructs,

and it would be a bad idea to use two

variables with such similar names.

This last point is perhaps the most important:

Variable names in PHP are case-sensitive.

Using the wrong letter case is a very

common cause of bugs. [If you used, for

example,

$_server

$_Server

in the

previous script, you’d see either an error

message or nothing at all A.]

A Misspelling a variable’s name, including its

case, will create undesired and unpredictable

results.

ptg18144795

Variables 37

To help minimize bugs, I recommend the

following policies:

Always use all lowercase variable names.

Make your variable names descriptive

[for example,

$first_name

is better

than

$fn

Use comments to indicate the purpose

of variables [Script 2.2], redundant as

that may seem.

Above all, be consistent with whatever

naming convention you choose!

Table 2.1 lists some sample valid variables;

Table 2.2 lists some invalid variables and

the rules they violate.

Unlike some other languages, PHP

doesn’t require you to declare or initialize a

variable prior to use, although PHP does issue

warnings when you do. In other words, you

can refer to variables without first defining

them. But it’s best not to do that; try to write

scripts so that every variable is defined or

validated before use.

There are two main variable naming

conventions, determined by how you delineate

words. These are the so-called camel-hump

or camel-case [named because of the way

capital letters break up the word—for example,

$FirstName

] and underscore [

$first_name

]

styles. This book uses the latter convention.

Script 2.2 Properly documenting the purposes of

variables, along with using meaningful names, is a

hallmark of a professional programmer.

5 Variables and Comments

TABLE 2.1 Valid Variables in PHP

Name

$first_name

$person

$address1

$_SERVER

TABLE 2.2 Invalid Variables in PHP

Name Reason

$first name

Has a space

$first.name

Has a period

first_name

Does not begin with

$1address

A number cannot follow

ptg18144795

38 Chapter 2

Types of Variables

This book covers three common PHP vari-

able types:

numbers

strings

, and

arrays

This chapter introduces them quickly, and

later chapters discuss them in more detail:

Chapter 4, “Using Numbers”

Chapter 5, “Using Strings”

Chapter 7, “Using Arrays”

A fourth variable type,

objects

, is intro-

duced in Appendix B, “Resources and Next

Steps,” but isn’t covered in this book. That

particular subject is just too advanced for

a beginner’s guide—in fact, basic coverage

of the subject in my

PHP Advanced and

Object-Oriented Programing: Visual

QuickPro Guide, Third Edition

[Peachpit

Press, 2013] requires over 150 pages!

TABLE 2.3 Valid Numbers in PHP

Number Type

1 Integer

1.0 Floating-point

1972 Integer

19.72 Floating-point

–1 Integer

–1.0 Floating-point

TABLE 2.4 Invalid Numbers in PHP

Number Reason

1/3Contains a slash

1996aContains a letter

08.02.06 Contains multiple

decimals

Numbers

Technically speaking, PHP breaks numbers

into two types:

integers

and

floating-point

[also known as

double-precision

floating-

point

doubles

]. Due to the lax way PHP

handles variables, it largely won’t affect your

programming to group the two categories

of numbers into one all-inclusive member-

ship, at least when you’re just starting out.

Still, let’s briefly discuss the differences

between the two, to be precise.

The first type of numbers—

integers

—is also

known as

whole numbers

. They can be pos-

itive or negative but include neither fractions

nor decimals. Numbers that use a decimal

point [even something like 1.0] are

floating-

point

numbers, also known as

floats

. You

use floating-point numbers to refer to frac-

tions, because the only way to express a

fraction in PHP is to convert it to its decimal

equivalent. Hence, 1¼ is written as 1.25.

Table 2.3 lists some sample valid numbers

and their formal type; Table 2.4 lists invalid

numbers and the rules they violate.

As you’ll soon see, you add quotation

marks around invalid numbers to turn them

into valid strings.

ptg18144795

Variables 39

Strings

A string is any number of characters

enclosed within a pair of either single [

]

or double [

] quotation marks. Strings can

contain any combination of characters

that exist: letters, numbers, symbols, and

spaces. Strings can also contain variables.

Here are examples of valid string values:

"Hello, world!"

"Hello, $first_name!"

"1/3"

'Hello, world! How are you today?'

"08.02.06"

"1996"

That last example is an

empty string

—a

string that contains no characters.

To create a string, just wrap 0 or more

characters within quotation marks. There

are cases, however, where you may run

into problems. For example:

"I said, "How are you?""

This string will be tricky. Chapter 1, “Getting

Started with PHP,” hinted at the same prob-

lem with respect to printing HTML code.

When PHP hits the second quotation mark

in the example, it assumes the string ends

there; the continuing text [

How…

] causes

an error. To use a quotation mark within a

string you

escape

the quotation mark by

putting a backslash [

] before it:

"I said, \"How are you?\""

The backslash tells PHP to treat each

escaped quotation mark as part of the

value

of the string, rather than using it as

the string’s opening or closing indicators.

You can similarly circumvent this problem

by using different quotation mark types:

'I said, "How are you?"'

"I said, 'How are you?'"

Notice that “1996” converts an integer

into a string, simply by placing the number

within quotes. Essentially, the string contains

the characters 1996, whereas the number [a

nonquoted value] would be equal to 1996. It’s

a fine distinction, and one that won’t matter

in your code, because PHP lets you perform

mathematical calculations with the string 1996

just as you can with the number.

Chapter 1 also demonstrated how to

create a new line by printing the

charac-

ter within double quotation marks. Although

escaping a quotation mark prints the quota-

tion mark, escaping an n prints a new line,

escaping an r creates a carriage return, and

escaping a t creates a tab.

Understanding strings, variables, and the

single and double quotation marks is critical

to programming with PHP. For this reason, a

section at the end of this chapter is dedicated

to the subject.

ptg18144795

40 Chapter 2

Arrays

Arrays are covered more thoroughly in

Chapter 7, but let’s look at them briefly

here. Whereas a string or a number con-

tains a single value [both are said to be

scalar

], an array can have more than one

value assigned to it. You can think of an

array as a list or table of values: You can

put multiple strings and/or numbers into

one array.

Arrays use

keys

to create and retrieve the

values they store. The resulting structure—

a list of key-value pairs—is similar to a

two-column spreadsheet. Unlike arrays in

other programming languages, the array

structure in PHP is so flexible that it can use

either numbers or strings for both the keys

and the values. The array doesn’t even need

to be consistent in this respect. [All of this

will make more sense in Chapter 7, when

you start working with specific examples.]

PHP supports two kinds of arrays, based

on the format of the keys. If the array uses

numbers for the keys [Table 2.5], it’s known

as an

indexed

array. If it uses strings for

the keys [Table 2.6], it’s an

associative

array. In either case, the values in the array

can be of any variable type [string, number,

and so on].

The array’s key is also referred to as

its index. You’ll see these two terms used

interchangeably.

An array can, and frequently will, contain

other arrays, creating what is called a multi-

dimensional array.

What PHP refers to as an associative

array is known as a hash in Perl and Ruby,

among other languages.

TABLE 2.5 Indexed Array

Key Value

0 Dev

1 Rachel

2 Denise

3 Arnold

TABLE 2.6 Associative Array

Key Value

VT Vermont

NHNew Hampshire

IA Iowa

PA Pennsylvania

ptg18144795

Variables 41

Variable Values

To assign a value to a variable, regardless

of the variable type, you use the equals

sign [

]. Therefore, the equals sign is known

as the

assignment operator

, because it

assigns the value on the right to the variable

on the left. For example:

$number = 1;

$floating_number = 1.2;

$string = "Hello, world!";

Each of these lines represents a complete

statement [that is, an executable action],

so each concludes with a semicolon.

To print the value of a variable, use the

function:

print $number;

print $string;

If you want to print a variable’s value within

a context, you can place the variable’s

name in the printed string, as long as you

use double quotation marks A:

print "Number is $number";

print "String is $string";

Using

in this way works for the sca-

lar [single-valued] variable types—numbers

and strings. For complex variable types—

arrays and objects—you cannot just use

print "_SERVER is $_SERVER";

As you’ve already seen,

print_r[]

can

handle these nonscalar types, and you’ll

learn other approaches later in the book.

Whether you’re dealing with scalar or non-

scalar variables, don’t forget that printing

out their values is an excellent debugging

technique when you’re having problems

with a script.

A The result of printing the values

of two variables.

B Using the

statement on a complex

variable type, such as an array, will not have the

results you desire.

ptg18144795

42 Chapter 2

Because variable types aren’t locked in

[PHP is referred to as a

weakly typed

lan-

guage], they can be changed on the fly:

$variable = 1;

$variable = "Greetings";

If you were to print the value of

$variable

now, the result would be

Greetings

. The

following section better demonstrates the

concept of assigning values to variables

and then accessing those values.

To assign values to and

access variables:

1. Create a new PHP script in your

text editor or IDE, to be named

variables.php

[Script 2.3].

2. Create the initial HTML tags:

Variables

3. Begin the PHP code:

ptg18144795

Variables 43

Remember that each statement must

conclude with a semicolon and that the

variable names are case-sensitive.

5. Print out the values of the variables

within some context:

print "

The address is:

➝

$street
$city $state

➝

$zip

Here a single

statement refer-

ences all the variables. The entire string

to be printed [consisting of text, HTML

tags, and variables] is enclosed within

double quotation marks. The HTML

tags make the text flow over mul-

tiple lines in the browser.

6. Complete the PHP section and the

HTML page:

7. Save the file as

variables.php

, upload

it to your server [or save it to the appro-

priate directory on your computer], and

test it in your browser C.

If you see a parse error D when you

run this script, you probably either omitted

a semicolon or have an imbalance in your

quotation marks. In such particular cases, the

mistake itself is likely on the previous line of

code [than reported in the error message] but

wasn’t caught by PHP until the next line.

If one of the variable’s values isn’t

printed out or you see an Undefined variable

error E, you most likely failed to spell a

variable name the same way twice.

If you see a blank page, you most likely

have an error but PHP’s

display_errors

configuration is set to off. See Chapter 3,

“HTML Forms and PHP,” for details.

C Some variables are assigned values, and then

printed within a context.

D Parse errors are the most common type of PHP

error, as you’ll discover. They’re frequently caused

by missing semicolons or mismatched quotation

marks or parentheses.

E The

Undefined variable

error indicates that

you used a variable with no value [it hasn’t been

defined]. This can happen with misspellings and

capitalization inconsistencies.

ptg18144795

44 Chapter 2

Understanding

Quotation Marks

Now that you know the basics of variables

and how to create them, let’s do an exer-

cise to make sure you completely under-

stand how to properly use quotation marks.

PHP, like most programming languages,

allows you to use both double [

] and

single [

] quotation marks—but they give

vastly different results. It’s critical that you

comprehend the distinction, so the next

example will run tests using both types just

to emphasize the different behaviors.

The rule to remember is:

Items within

single quotation marks are treated literally;

items within double quotation marks

are extrapolated

. This means that within

double quotation marks, a variable’s name

is replaced with its value, as in Script 2.3,

but the same is not true for single quota-

tion marks.

This rule applies anywhere in PHP you

might use quotation marks, including

uses of the

function and the assign-

ment of values to string variables. An

example is the best way to demonstrate

this critical concept.

ptg18144795

Variables 45

To use quotation marks:

1. Begin a new PHP script in your text

editor or IDE, to be named

quotes.php

[Script 2.4].

2. Create the initial HTML tags:

Quotes

3. Begin the PHP code:

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