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 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 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 //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
