PHP 手冊
上一頁 |  | 下一頁

# 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 ?>`

> 備注: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.

上一頁 | 最開頭 | 下一頁
is_string | 上一層 | print_r