defined

PHP 手册
后退		前进

Description

bool defined ( string name )

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

例子 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 ; } ?>

注: 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 .