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defined

(PHP 3, 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.

注意: 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

例 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()
constant()
get_defined_constants()
function_exists()
The section on Constants



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