PHP define() Function

PHP | defined() function

Last Updated : 23 Aug, 2019

The PHP defined() function is an inbuilt function in PHP which checks whether a constant is exists or not, in other words, defined or not. Syntax:

bool defined($constant_name);

Parameter: This function accepts a single parameter as mentioned above and described below.

$constant_name: This is required parameter. It specifies the name of the constant.

Return Value: This function returns TRUE if constant exists and FALSE otherwise. Note: This function is available for PHP 4.0.0 and newer version. Below examples illustrate the function: Example 1:

php

<?php
define("constant_key", "value for the constant key");
echo defined("constant_key");
?>

Output:

Example 2: checking with if condition after defining the constant.

php

<?php
define("constant_key", "value for the constant key");
if(defined("constant_key")){
    echo "constant_key is defined";
}else{
    echo "constant_key is not defined";
}
?>

Output:

constant_key is defined

Example 3: checking with if condition without defining the constant.

php

<?php
//define("constant_key", "value for the constant key");
if(defined("constant_key")){
    echo "constant_key is defined";
}else{
    echo "constant_key is not defined";
}
?>

Output:

constant_key is not defined

PHP define() Function

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