Hướng dẫn php diamond question mark

This is a charset issue. As such, it can have gone wrong on many different levels, but most likely, the strings in your database are utf-8 encoded, and you are presenting them as iso-8859-1. Or the other way around.

The proper way to fix this problem, is to get your character-sets straight. The simplest strategy, since you're using PHP, is to use iso-8859-1 throughout your application. To do this, you must ensure that:

  • All PHP source-files are saved as iso-8859-1 (Not to be confused with cp-1252).
  • Your web-server is configured to serve files with charset=iso-8859-1
  • Alternatively, you can override the webservers settings from within the PHP-document, using header.
  • In addition, you may insert a meta-tag in you HTML, that specifies the same thing, but this isn't strictly needed.
  • You may also specify the accept-charset attribute on your
  • Database tables are defined with encoding as latin1
  • The database connection between PHP to and database is set to latin1

If you already have data in your database, you should be aware that they are probably messed up already. If you are not already in production phase, just wipe it all and start over. Otherwise you'll have to do some data cleanup.

A note on meta-tags, since everybody misunderstands what they are:

When a web-server serves a file (A HTML-document), it sends some information, that isn't presented directly in the browser. This is known as HTTP-headers. One such header, is the Content-Type header, which specifies the mimetype of the file (Eg. text/html) as well as the encoding (aka charset). While most webservers will send a Content-Type header with charset info, it's optional. If it isn't present, the browser will instead interpret any meta-tags with http-equiv="Content-Type". It's important to realise that the meta-tag is only interpreted if the webserver doesn't send the header. In practice this means that it's only used if the page is saved to disk and then opened from there.

This page has a very good explanation of these things.

If you ever come across this issue where special characters like aprostrophe's and ñ's are being replaced by this weird symbol, �, it could be a char-set thing.

Hướng dẫn php diamond question mark

I tried a couple of solutions but none of them worked, until I looked into my php.ini for anything related to setting the character encoding, and I found that default_charset is set to UTF-8.

I simply changed it to ISO-8859-1, restarted the IIS server, and refreshed the page.

Hướng dẫn php diamond question mark

And it worked!

Php.ini can be found on your PHP installation folder, in my case: 

C:\Program Files (x86)\PHP\php-7.4.8\php.ini

Set default_charset to ISO-8859-1 and enable it by removing the ; in front

default_charset = "iso-8859-1"


Comparison operators, as their name implies, allow you to compare two values. You may also be interested in viewing the type comparison tables, as they show examples of various type related comparisons.

Comparison Operators
$a == $b Equal true if $a is equal to $b after type juggling.
$a === $b Identical true if $a is equal to $b, and they are of the same type.
$a != $b Not equal true if $a is not equal to $b after type juggling.
$a <> $b Not equal true if $a is not equal to $b after type juggling.
$a !== $b Not identical true if $a is not equal to $b, or they are not of the same type.
$a < $b Less than true if $a is strictly less than $b.
$a > $b Greater than true if $a is strictly greater than $b.
$a <= $b Less than or equal to true if $a is less than or equal to $b.
$a >= $b Greater than or equal to true if $a is greater than or equal to $b.
$a <=> $b Spaceship An int less than, equal to, or greater than zero when $a is less than, equal to, or greater than $b, respectively.

If both operands are numeric strings, or one operand is a number and the other one is a numeric string, then the comparison is done numerically. These rules also apply to the switch statement. The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.


Prior to PHP 8.0.0, if a string is compared to a number or a numeric string then the string was converted to a number before performing the comparison. This can lead to surprising results as can be seen with the following example:

var_dump(== "a");
var_dump("1" == "01");
var_dump("10" == "1e1");
var_dump(100 == "1e2");

switch (

"a") {

Output of the above example in PHP 7:


Output of the above example in PHP 8:


// Integers
echo <=> 1// 0
echo <=> 2// -1
echo <=> 1// 1

 // Floats

echo 1.5 <=> 1.5// 0
echo 1.5 <=> 2.5// -1
echo 2.5 <=> 1.5// 1

 // Strings

echo "a" <=> "a"// 0
echo "a" <=> "b"// -1
echo "b" <=> "a"// 1echo "a" <=> "aa"// -1
echo "zz" <=> "aa"// 1

 // Arrays

echo [] <=> []; // 0
echo [123] <=> [123]; // 0
echo [123] <=> []; // 1
echo [123] <=> [121]; // 1
echo [123] <=> [124]; // -1

 // Objects

$a = (object) ["a" => "b"]; 
$b = (object) ["a" => "b"]; 
$a <=> $b// 0$a = (object) ["a" => "b"]; 
$b = (object) ["a" => "c"]; 
$a <=> $b// -1$a = (object) ["a" => "c"]; 
$b = (object) ["a" => "b"]; 
$a <=> $b// 1

 // not only values are compared; keys must match

$a = (object) ["a" => "b"]; 
$b = (object) ["b" => "b"]; 
$a <=> $b// 1?>

For various types, comparison is done according to the following table (in order).

Comparison with Various Types
Type of Operand 1Type of Operand 2Result
null or string string Convert null to "", numerical or lexical comparison
bool or null anything Convert both sides to bool, false < true
object object Built-in classes can define its own comparison, different classes are incomparable, same class see Object Comparison
string, resource, int or float string, resource, int or float Translate strings and resources to numbers, usual math
array array Array with fewer members is smaller, if key from operand 1 is not found in operand 2 then arrays are incomparable, otherwise - compare value by value (see following example)
object anything object is always greater
array anything array is always greater

Example #1 Boolean/null comparison

// Bool and null are compared as bool always
var_dump(== TRUE);  // TRUE - same as (bool)1 == TRUE
var_dump(== FALSE); // TRUE - same as (bool)0 == FALSE
var_dump(100 TRUE); // FALSE - same as (bool)100 < TRUE
var_dump(-10 FALSE);// FALSE - same as (bool)-10 < FALSE
var_dump(min(-100, -10NULL10100)); // NULL - (bool)NULL < (bool)-100 is FALSE < TRUE

Example #2 Transcription of standard array comparison

// Arrays are compared like this with standard comparison operators as well as the spaceship operator.
function standard_array_compare($op1$op2)
    if (
count($op1) < count($op2)) {
        return -
1// $op1 < $op2
} elseif (count($op1) > count($op2)) {
1// $op1 > $op2
    foreach (
$op1 as $key => $val) {
        if (!
array_key_exists($key$op2)) {
        } elseif (
$val $op2[$key]) {
            return -
        } elseif (
$val $op2[$key]) {
0// $op1 == $op2


Comparison of floating point numbers

Because of the way floats are represented internally, you should not test two floats for equality.

See the documentation for float for more information.

Note: Be aware that PHP's type juggling is not always obvious when comparing values of different types, particularly comparing ints to bools or ints to strings. It is therefore generally advisable to use === and !== comparisons rather than == and != in most cases.

Incomparable Values

While identity comparison (=== and !==) can be applied to arbitrary values, the other comparison operators should only be applied to comparable values. The result of comparing incomparable values is undefined, and should not be relied upon.

Ternary Operator

Another conditional operator is the "?:" (or ternary) operator.

Example #3 Assigning a default value

// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' $_POST['action'];// The above is identical to this if/else statement
if (empty($_POST['action'])) {
$action 'default';
} else {
$action $_POST['action'];

The expression (expr1) ? (expr2) : (expr3) evaluates to expr2 if expr1 evaluates to true, and expr3 if expr1 evaluates to false.

It is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 evaluates to the result of expr1 if expr1 evaluates to true, and expr3 otherwise. expr1 is only evaluated once in this case.

Note: Please note that the ternary operator is an expression, and that it doesn't evaluate to a variable, but to the result of an expression. This is important to know if you want to return a variable by reference. The statement return $var == 42 ? $a : $b; in a return-by-reference function will therefore not work and a warning is issued.


It is recommended to avoid "stacking" ternary expressions. PHP's behaviour when using more than one unparenthesized ternary operator within a single expression is non-obvious compared to other languages. Indeed prior to PHP 8.0.0, ternary expressions were evaluated left-associative, instead of right-associative like most other programming languages. Relying on left-associativity is deprecated as of PHP 7.4.0. As of PHP 8.0.0, the ternary operator is non-associative.

Example #4 Non-obvious Ternary Behaviour

// on first glance, the following appears to output 'true'
echo (true 'true' false 't' 'f');// however, the actual output of the above is 't' prior to PHP 8.0.0
// this is because ternary expressions are left-associative

// the following is a more obvious version of the same code as above

echo ((true 'true' false) ? 't' 'f');// here, one can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool)true, thus returning the true branch of the
// second ternary expression.


Chaining of short-ternaries (?:), however, is stable and behaves reasonably. It will evaluate to the first argument that evaluates to a non-falsy value. Note that undefined values will still raise a warning.

Example #5 Short-ternary chaining

echo ?: ?: ?: 3PHP_EOL//1
echo ?: ?: ?: 3PHP_EOL//2
echo ?: ?: ?: 3PHP_EOL//3

Null Coalescing Operator

Another useful shorthand operator is the "??" (or null coalescing) operator.

Example #6 Assigning a default value

// Example usage for: Null Coalesce Operator
$action $_POST['action'] ?? 'default';// The above is identical to this if/else statement
if (isset($_POST['action'])) {
$action $_POST['action'];
} else {
$action 'default';

The expression (expr1) ?? (expr2) evaluates to expr2 if expr1 is null, and expr1 otherwise.

In particular, this operator does not emit a notice or warning if the left-hand side value does not exist, just like isset(). This is especially useful on array keys.

Note: Please note that the null coalescing operator is an expression, and that it doesn't evaluate to a variable, but to the result of an expression. This is important to know if you want to return a variable by reference. The statement return $foo ?? $bar; in a return-by-reference function will therefore not work and a warning is issued.


The null coalescing operator has low precedence. That means if mixing it with other operators (such as string concatenation or arithmetic operators) parentheses will likely be required.

// Raises a warning that $name is undefined.
print 'Mr. ' $name ?? 'Anonymous';// Prints "Mr. Anonymous"
print 'Mr. ' . ($name ?? 'Anonymous');


Please note that the null coalescing operator allows for simple nesting:

Example #7 Nesting null coalescing operator


$bar null;
$baz 1;
$qux 2;


$foo ?? $bar ?? $baz ?? $qux// outputs 1?>

crazy888s at hotmail dot com

12 years ago

I couldn't find much info on stacking the new ternary operator, so I ran some tests:

echo 0 ?: 1 ?: 2 ?: 3; //1
echo 1 ?: 0 ?: 3 ?: 2; //1
echo 2 ?: 1 ?: 0 ?: 3; //2
echo 3 ?: 2 ?: 1 ?: 0; //3echo 0 ?: 1 ?: 2 ?: 3; //1
echo 0 ?: 0 ?: 2 ?: 3; //2
echo 0 ?: 0 ?: 0 ?: 3; //3

It works just as expected, returning the first non-false value within a group of expressions.

niall at maranelda dot org

4 years ago

Care must be taken when using the spaceship operator with arrays that do not have the same keys:

- Contrary to the notes above ("Example #2 Transcription of standard array comparison"), it does *not* return null if the left-hand array contains a key that the right-hand array does not.
- Because of this, the result depends on the order you do the comparison in.

For example:

$a = ['a' => 1, 'b' => 2, 'c' => 3, 'e' => 4];
$b = ['a' => 1, 'b' => 2, 'd' => 3, 'e' => 4];var_dump($a <=> $b);        // int(1) : $a > $b because $a has the 'c' key and $b doesn't.var_dump($b <=> $a);        // int(1) : $b > $a because $b has the 'd' key and $a doesn't.

adam at caucho dot com

16 years ago

Note: according to the spec, PHP's comparison operators are not transitive.  For example, the following are all true in PHP5:

"11" < "a" < 2 < "11"

As a result, the outcome of sorting an array depends on the order the elements appear in the pre-sort array.  The following code will dump out two arrays with *different* orderings:

$a = array(2,    "a""11", 2);
$b = array(2,    "11", "a"2);

This is not a bug report -- given the spec on this documentation page, what PHP does is "correct".  But that may not be what was intended...

rshawiii at yahoo dot com

16 years ago

You can't just compare two arrays with the === operator
like you would think to find out if they are equal or not.  This is more complicated when you have multi-dimensional arrays.  Here is a recursive comparison function.

* Compares two arrays to see if they contain the same values.  Returns TRUE or FALSE.
* usefull for determining if a record or block of data was modified (perhaps by user input)
* prior to setting a "date_last_updated" or skipping updating the db in the case of no change.
* @param array $a1
* @param array $a2
* @return boolean
function array_compare_recursive($a1, $a2)
   if (!(
is_array($a1) and (is_array($a2)))) { return FALSE;}   

       if (!

count($a1) == count($a2))
FALSE; // arrays don't have same number of entries

         foreach (

$a1 as $key => $val)
       if (!
array_key_exists($key, $a2))
FALSE; // uncomparable array keys don't match
       elseif (
is_array($val) and is_array($a2[$key]))  // if both entries are arrays then compare recursive
{if (!array_compare_recursive($val,$a2[$key])) return FALSE;
       elseif (!(
$val === $a2[$key])) // compare entries must be of same type.
{return FALSE;
TRUE; // $a1 === $a2


15 years ago

When you want to know if two arrays contain the same values, regardless of the values' order, you cannot use "==" or "===".  In other words:

(array(1,2) == array(2,1)) === false;

To answer that question, use:

function array_equal($a, $b) {
    return (
is_array($a) && is_array($b) && array_diff($a, $b) === array_diff($b, $a));

A related, but more strict problem, is if you need to ensure that two arrays contain the same key=>value pairs, regardless of the order of the pairs.  In that case, use:

function array_identical($a, $b) {
    return (
is_array($a) && is_array($b) && array_diff_assoc($a, $b) === array_diff_assoc($b, $a));

$a = array (2, 1);
$b = array (1, 2);
// true === array_equal($a, $b);
// false === array_identical($a, $b);
$a = array ('a' => 2, 'b' => 1);
$b = array ('b' => 1, 'a' => 2);
// true === array_identical($a, $b)
// true === array_equal($a, $b)

(See also the solution "rshawiii at yahoo dot com" posted)

Sumon Mahmud

2 years ago

Extending from here: https://www.php.net/manual/en/language.operators.comparison.php#121907

$a = ['a' => 1, 'b' => 2, 'c' => 3, 'e' => 4];
$b = ['a' => 1, 'b' => 2, 'd' => 3, 'e' => 4];

echo $a > $b; // 0
echo $b > $a; // 0
echo $a <$b; // 0
echo $b < $a; // 0

If using spaceship operator then it is returning true like :

echo $a <=> $b; //1
echo $b <=> $a; //1
echo $a <=> $b; //1
echo $b <=> $a; //1

Cuong Huy To

11 years ago

In the table "Comparison with Various Types", please move the last line about "Object" to be above the line about "Array", since Object is considered to be greater than Array (tested on 5.3.3)

(Please remove my "Anonymous" post of the same content before. You could check IP to see that I forgot to type my name)

Ryan Mott

3 years ago

Searching for "double question mark" operator should find this page (and hopefully after this comment the crawlers will agree)

Marcin Kuzawiski

7 years ago

A < B and still B < A...

$A = [1 => 1, 2 => 0, 3 => 1];
$B = [1 => 1, 3 => 0, 2 => 1];

var_dump($A < $B);  // TRUE
var_dump($B < $A);  // TRUE

var_dump($A > $B);  // TRUE
var_dump($B > $A);  // TRUE

Next - C and D are comparable, but neither C < D nor D < C (and still C != D)...

$C = [1 => 1, 2 => 1, 3 => 0];
$D = [1 => 1, 3 => 1, 2 => 0];

var_dump($C < $D); // FALSE
var_dump($D < $C); // FALSE

var_dump($C > $D); // FALSE
var_dump($D > $C); // FALSE

var_dump($D == $C); // FALSE

admin at zeros dot co dot id

1 month ago

Please be careful when you try to compare strings that have a plus sign `+` at the beginning (such as phone number, etc). When you use the Equal operator `==` PHP will ignore the plus sign. Use Identical operator `===` instead


$str1 = "62";
$str2 = "+62";

var_dump($str1 == $str2); // bool(true)
var_dump($str1 === $str2); // bool(false)


1 year ago

Very careful when reading PHP documentation, Here's a lot of miss information.

According to documentation, They say's (int) 0 == (string) "a" is true. But it is not in PHP 8.

var_dump(0 == "a"); // 0 == 0 -> true

Now In PHP 8 it's False.