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
/* 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;
}
?>

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