defined

Podręcznik PHP
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Description

bool defined ( string name )

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

Przykład 1. Checking Constants
<?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; } ?>

Notatka: 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.