(PHP 4, PHP 5, PHP 7)
defined — Checks whether a given named constant exists
defined ( string $name ) : bool
Checks whether the given constant exists and is defined.
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().
name
The constant name.
Returns true if the named constant given by name has been defined, false otherwise.
Example #1 Checking Constants
<?php
/* Note the use of quotes, this is important. This example is checking
* if the string 'TEST' is the name of a constant named TEST */
if (defined('TEST')) {
echo TEST;
}
?>
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https://www.php.net/manual/en/function.defined.php