defined
(PHP 4, PHP 5)
defined — Checks whether a given named constant exists
说明
bool defined ( string $name )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().
参数
- name
-
The constant name.
返回值
Returns TRUE if the named constant given by name has been defined, FALSE otherwise.
范例
Example #1 Checking Constants
<?php
if (defined('TEST')) {
echo TEST;
}
?>
参见
- define() - Defines a named constant
- constant() - Returns the value of a constant
- get_defined_constants() - Returns an associative array with the names of all the constants and their values
- function_exists() - Return TRUE if the given function has been defined
- The section on Constants
