PHP | defined() function

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:

1

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