What is PHP defined() Function:
In PHP, defined() function is used to check whether a constant is defined or not. In your PHP script, before defining a constant it is safe to check whether it has already been defined or not to avoid issues.
Syntax:
defined(CONSTANT_NAME)
Parameters:
The function has one required parameter –
CONSTANT_NAME (Required): Name of the constant.
Return Values:
The function returns-
- TRUE – if the constant has been defined.
- FALSE – if the constant hasn’t been defined.
Examples:
Example 1:
<?php
define ("LANGUAGE", "php");
var_dump( defined("LANGUAGE"));
?>
Output:
bool(true)
Example 2:
<?php
if( !defined("SITENAME")) define("SITENAME", "Schools of Web");
echo 'Site name is: "' . SITENAME . '"';
?>
Output:
Site name is: “Schools of Web”
Explanation:
Before defining the constant SITENAME, we checked whether it has been defined by defined() function.
Practical Usages of PHP defined() Function:
To avoid collision, before defining a constant, you can use defined() function to check whether it has been defined already.
PHP Version Support:
PHP 4, PHP 5, PHP 7, PHP 8
Summary: PHP defined() Function
PHP defined() function is built-in misc. function. In almost all your PHP application, you’ll always define some constants. And, defined() function facilitates you to check the existence of a constant.