defined

Checks whether a given named constant exists (PHP 3, PHP 4 )
bool defined ( string name )

Returns TRUE if the named constant given by name has been defined, FALSE otherwise.

Example 567. Checking Constants

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<?php
/* Note the use of quotes, this is important.  This example is checking
* if the string 'CONSTANT' is the name of a constant named CONSTANT */
if (defined('CONSTANT')) {
   echo CONSTANT;
}
?>
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().

See also define(), constant(), get_defined_constants(), function_exists(), and the section on Constants.

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