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# isset

(PHP 3, PHP 4, PHP 5 )

isset -- Determine whether a variable is set

## Description

bool isset ( mixed var [, mixed var [, ...]] )

Returns `TRUE` if var exists; `FALSE` otherwise.

If a variable has been unset with unset(), it will no longer be set. isset() will return `FALSE` if testing a variable that has been set to `NULL`. Also note that a `NULL` byte ("\0") is not equivalent to the PHP `NULL` constant.

> Warning: isset() only works with variables as passing anything else will result in a parse error. For checking if constants are set use the defined() function.

`<?php
$var = '';
// This will evaluate to TRUE so the text will be printed.
if (isset($var)) {
echo "This var is set set so I will print.";
}
// In the next examples we'll use var_dump to output
// the return value of isset().
$a = "test";
$b = "anothertest";
var_dump(isset($a)); // TRUE
var_dump(isset($a, $b)); // TRUE
unset ($a);
var_dump(isset($a)); // FALSE
var_dump(isset($a, $b)); // FALSE
$foo = NULL;
var_dump(isset($foo)); // FALSE
?>`

This also work for elements in arrays:

`<?php $a = array ('test' => 1, 'hello' => NULL); var_dump(isset($a['test'])); // TRUE var_dump(isset($a['foo'])); // FALSE var_dump(isset($a['hello'])); // FALSE // The key 'hello' equals NULL so is considered unset // If you want to check for NULL key values then try: var_dump(array_key_exists('hello', $a)); // TRUE ?>`

> 注: 由於這是一個語言結構、而不是函數，所以不能以變數函數的方式來呼叫。

See also empty(), unset(), defined(), the type comparison tables, array_key_exists(), and the error control @ operator.

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