Hàm file_exists[] sẽ kiểm tra xem file hoặc thư mục có tồn tại hay không.
Bài viết này được đăng tại freetuts.net, không được copy dưới mọi hình thức.
Cú pháp
Cú pháp: file_exists[ $filename];
Trong đó:
$filenamelà đường dẫn tới file hoặc thư mục cần kiểm tra.
Kết quả trả về
Hàm sẽ trả về True nếu file, thư mục truyền vào tồn tại. Ngược lại hàm sẽ trả về False.
Bài viết này được đăng tại [free tuts .net]
Ví dụ
Ví dụ về cách sử dụng hàm file_exists[]:
Code
file_put_contents['test.txt', 'this is a test'];
if [file_exists['test.txt']] {
echo "file tồn tại!";
}else{
echo "file không tồn tồn tại!";
}Tham khảo: php.net
[PHP 4, PHP 5, PHP 7, PHP 8]
defined — Checks whether a given named constant exists
Description
defined[string $constant_name]: bool
Note:
If you want to see if a variable exists, use isset[] as defined[] only applies to constants. If you want to see if a function exists, use function_exists[].
Parameters
constant_nameThe constant name.
Return Values
Returns true if the named constant given by constant_name has been defined, false otherwise.
Examples
Example #1 Checking Constants
See Also
- define[] - Defines a named constant
- constant[] - Returns the value of a constant
- get_defined_constants[] - Returns an associative array with the names of all the constants and their values
- function_exists[] - Return true if the given function has been defined
- The section on Constants
daniel at neville dot tk ¶
14 years ago
My preferred way of checking if a constant is set, and if it isn't - setting it [could be used to set defaults in a file, where the user has already had the opportunity to set their own values in another.]
Dan.
ASchmidt at Anamera dot net ¶
5 years ago
// Checking the existence of a class constant, if the class is referenced by a variable.
class Class_A
{
const CONST_A = 'value A';
}
// When class name is known.
if [ defined[ 'Class_A::CONST_A' ] ]
echo 'Class_A::CONST_A defined';
// Using a class name variable. Note the double quotes.
$class_name = Class_A::class;
if [ defined[ "$class_name::CONST_A" ] ]
echo '$class_name::CONST_A defined';
// Using an instantiated object for a variable class.
$object_A = new $class_name[];
if [ defined[ get_class[$object_A].'::CONST_A' ] ]
echo '$object_A::CONST_A defined';
tris+php at tfconsulting dot com dot au ¶
13 years ago
Before using defined[] have a look at the following benchmarks:
true 0.65ms
$true 0.69ms [1]
$config['true'] 0.87ms
TRUE_CONST 1.28ms [2]
true 0.65ms
defined['TRUE_CONST'] 2.06ms [3]
defined['UNDEF_CONST'] 12.34ms [4]
isset[$config['def_key']] 0.91ms [5]
isset[$config['undef_key']] 0.79ms
isset[$empty_hash[$good_key]] 0.78ms
isset[$small_hash[$good_key]] 0.86ms
isset[$big_hash[$good_key]] 0.89ms
isset[$small_hash[$bad_key]] 0.78ms
isset[$big_hash[$bad_key]] 0.80ms
PHP Version 5.2.6, Apache 2.0, Windows XP
Each statement was executed 1000 times and while a 12ms overhead on 1000 calls isn't going to have the end users tearing their hair out, it does throw up some interesting results when comparing to if[true]:
1] if[$true] was virtually identical
2] if[TRUE_CONST] was almost twice as slow - I guess that the substitution isn't done at compile time [I had to double check this one!]
3] defined[] is 3 times slower if the constant exists
4] defined[] is 19 TIMES SLOWER if the constant doesn't exist!
5] isset[] is remarkably efficient regardless of what you throw at it [great news for anyone implementing array driven event systems - me!]
May want to avoid if[defined['DEBUG']]...
r dot hartung at roberthartung dot de ¶
12 years ago
You can use the late static command "static::" withing defined as well. This example outputs - as expected - "int [2]"
Lars Lernestal ¶
10 years ago
if you want to check id a class constant is defined use self:: before the constant name:
Shaun H ¶
14 years ago
I saw that PHP doesn't have an enum function so I created my own. It's not necessary, but can come in handy from time to time.
passerbyxp at gmail dot com ¶
9 years ago
This function, along with constant[], is namespace sensitive. And it might help if you imagine them always running under the "root namespace":