isset

(PHP 4, PHP 5, PHP 7, PHP 8)

isset — 変数が宣言されていること、そして null とは異なることを検査する

説明

isset(mixed $var, mixed ...$vars): bool

変数がセットされているかを調べます。これは変数が宣言されており、 null とは異なる値だということです。

変数が、unset() により割当を解除された場合、もはやセットされているとは見做されません。

null が代入されている変数をチェックしたときは、 isset() は false を返します。 null文字 ("\0") は PHP の null 定数とは異なることにも注意してください。

複数のパラメータが渡された場合は、 isset() は全てのパラメータがセットされている場合にだけ true を返します。評価は左から右へと行われ、セットされていない値に出会うとすぐに評価が止まります。

パラメータ

var: 調べたい変数。
vars: 別の変数。

戻り値

var が存在して null 以外の値をとれば true、そうでなければ false を返します。

例

例1 isset() の例

<?php

$var = '';

// これは TRUE と評価されるので、テキストが出力される
if (isset($var)) {
    echo "This var is set so I will print.";
}

// 次の例では、isset() の戻り値を出力するため var_dump を
// 使用している

$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

?>

この関数は配列の要素に対しても動作します:

<?php

$a = array ('test' => 1, 'hello' => NULL, 'pie' => array('a' => 'apple'));

var_dump(isset($a['test']));            // TRUE
var_dump(isset($a['foo']));             // FALSE
var_dump(isset($a['hello']));           // FALSE

// キー 'hello' は NULL と等しいのでセットされていないと見なされる。
// もし NULL なキー値に対してチェックを行いたい場合、次を試してみること:
var_dump(array_key_exists('hello', $a)); // TRUE

// 配列の深い要素の値のチェック
var_dump(isset($a['pie']['a']));        // TRUE
var_dump(isset($a['pie']['b']));        // FALSE
var_dump(isset($a['cake']['a']['b']));  // FALSE

?>

例2 文字列のオフセットに対する isset()

<?php
$expected_array_got_string = 'somestring';
var_dump(isset($expected_array_got_string['some_key']));
var_dump(isset($expected_array_got_string[0]));
var_dump(isset($expected_array_got_string['0']));
var_dump(isset($expected_array_got_string[0.5]));
var_dump(isset($expected_array_got_string['0.5']));
var_dump(isset($expected_array_got_string['0 Mostel']));
?>

上の例の出力は以下となります。

bool(false)
bool(true)
bool(true)
bool(true)
bool(false)
bool(false)

注意

警告

isset() は何らかの値が渡された変数の場合のみ動作します。そうでない場合、パースエラーとなります。もし定数が設定されているかどうかをチェックする場合は、 defined() を使用してください。

注意: これは、関数ではなく言語構造のため、可変関数や名前付き引数を用いてコールすることはできません。

注意:
オブジェクトのアクセス不能なプロパティに対して isset() を使用した場合は、もしオーバーロードメソッド __isset() が宣言されていればそれをコールします。

参考

empty() - 変数が空であるかどうかを検査する
__isset()
unset() - 指定した変数の割当を解除する
defined() - 指定した名前の定数が存在するかどうかを調べる
PHP 型の比較表
array_key_exists() - 指定したキーまたは添字が配列にあるかどうかを調べる
is_null() - 変数が null かどうか調べる
エラー制御演算子 @

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User Contributed Notes 19 notes

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110

p_ignorethis_lbowers at gmail dot com ¶

8 years ago


I, too, was dismayed to find that isset($foo) returns false if ($foo == null). Here's an (awkward) way around it.

unset($foo);
if (compact('foo') != array()) {
  do_your_thing();
}

Of course, that is very non-intuitive, long, hard-to-understand, and kludgy. Better to design your code so you don't depend on the difference between an unset variable and a variable with the value null. But "better" only because PHP has made this weird development choice.

In my thinking this was a mistake in the development of PHP. The name ("isset") should describe the function and not have the desciption be "is set AND is not null". If it was done properly a programmer could very easily do (isset($var) || is_null($var)) if they wanted to check for this!

A variable set to null is a different state than a variable not set - there should be some easy way to differentiate. Just my (pointless) $0.02.

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kurdtpage at gmail dot com ¶

7 years ago


The new (as of PHP7) 'null coalesce operator' allows shorthand isset. You can use it like so:

<?php
// Fetches the value of $_GET['user'] and returns 'nobody'
// if it does not exist.
$username = $_GET['user'] ?? 'nobody';
// This is equivalent to:
$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

// Coalescing can be chained: this will return the first
// defined value out of $_GET['user'], $_POST['user'], and
// 'nobody'.
$username = $_GET['user'] ?? $_POST['user'] ?? 'nobody';
?>

Quoted from http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op

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eckoson at gmail dot com ¶

1 year ago


If you are annoyed by the behavior of isset() concerning null values, here is a handy function for you. its similar to array_key_exists but, its a lot more flexible and can check for multiple array keys across multiple arrays.
Not recursive!
Tested on php 8.1.6, linux
<?php
/**
* is_set
* @author DJ Eckoson
* @link @eckosongh Facebook Page
* checks whether variable names are set within the global space or they exists as an key and return if they are set (even if their values are null)
* @param string $var_name
*   name of the first variable to check
* @param array|null|string
*   optional array to check for key (if null, checks from $GLOBALS) OR
*   other variable names to check OR
*   other variable names and their associated arrays to their right (use null for global variables, optional if its the last parameter)
* check examples below
*/
function is_set(string $var_name, array|null|string ... $args): bool {
  $vars[$var_name] = null;
  if (array_key_exists(0, $args)) {
    if (is_array($args[0])) {
      $vars[$var_name] = $args[0];
    } elseif (is_string($args[0])) {
      goto main;
    }
    unset($args[0]);
  }
  main:
  if ($args) {
    $args = array_reverse($args);
    $cur_array = null;
    array_walk($args, function ($value) use (&$cur_array, &$vars): void {
      if (!is_string($value)) {
        $cur_array = $value;
      } else {
        $vars[$value] = $cur_array;
      }
    });
  }
  foreach ($vars as $name => $array) {
    if (!array_key_exists($name, $array??$GLOBALS)) return false;
  }
  return true;
}

// Examples
$arr1 = range(0, 5);
$arr2 = [
  'a' => 1,
  'b' => 2,
  'c' => 'hELLO wORLD!'
];
$gender = 'male';
$age = 12;
var_dump(is_set('age')); // true
var_dump(is_set('age', null)); // true
var_dump(is_set('age', $arr1)); // false
var_dump(is_set('age', array())); // false
var_dump(is_set('age', array('age' => 48))); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender')); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender', null)); // true

$c=$d=$e=$a=2;
$arr = [1,4];
var_dump(is_set('a', 'c', null, 0, 1, $arr)); // true
var_dump(is_set('a', 'c', null, 0, 4, $arr)); // false
?>
 Note:
 it won't work for variables declared locally inside a function;
 however you can use it for checking if array keys exists inside a function

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a dot schaffhirt at sedna-soft dot de ¶

16 years ago


You can safely use isset to check properties and subproperties of objects directly. So instead of writing

    isset($abc) && isset($abc->def) && isset($abc->def->ghi)

or in a shorter form

    isset($abc, $abc->def, $abc->def->ghi)

you can just write

    isset ($abc->def->ghi)

without raising any errors, warnings or notices.

Examples
<?php
    $abc = (object) array("def" => 123);
    var_dump(isset($abc));                // bool(true)
    var_dump(isset($abc->def));           // bool(true)
    var_dump(isset($abc->def->ghi));      // bool(false)
    var_dump(isset($abc->def->ghi->jkl)); // bool(false)
    var_dump(isset($def));                // bool(false)
    var_dump(isset($def->ghi));           // bool(false)
    var_dump(isset($def->ghi->jkl));      // bool(false)

    var_dump($abc);                       // object(stdClass)#1 (1) { ["def"] => int(123) }
    var_dump($abc->def);                  // int(123)
    var_dump($abc->def->ghi);             // null / E_NOTICE: Trying to get property of non-object
    var_dump($abc->def->ghi->jkl);        // null / E_NOTICE: Trying to get property of non-object
    var_dump($def);                       // null / E_NOTICE: Trying to get property of non-object
    var_dump($def->ghi);                  // null / E_NOTICE: Trying to get property of non-object
    var_dump($def->ghi->jkl);             // null / E_NOTICE: Trying to get property of non-object
?>

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yaogzhan at gmail dot com ¶

19 years ago


If you have 



<?PHP

class Foo

{

    protected $data = array('bar' => null);



    function __get($p)

    {

        if( isset($this->data[$p]) ) return $this->data[$p];

    }

}

?>



and

<?PHP

$foo = new Foo;

echo isset($foo->bar);

?>

will always echo 'false'. because the isset() accepts VARIABLES as it parameters, but in this case, $foo->bar is NOT a VARIABLE. it is a VALUE returned from the __get() method of the class Foo. thus the isset($foo->bar) expreesion will always equal 'false'.

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beuc at beuc dot net ¶

17 years ago


"empty() is the opposite of (boolean) var, except that no warning is generated when the variable is not set."

So essentially
<?php
if (isset($var) && $var)
?>
is the same as
<?php
if (!empty($var))
?>
doesn't it? :)

!empty() mimics the chk() function posted before.

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soywiz at php dot net ¶

18 years ago


Sometimes you have to check if an array has some keys. To achieve it you can use "isset" like this: isset($array['key1'], $array['key2'], $array['key3'], $array['key4'])
You have to write $array all times and it is reiterative if you use same array each time.

With this simple function you can check if an array has some keys:

<?php
function isset_array() {
    if (func_num_args() < 2) return true;
    $args = func_get_args();
    $array = array_shift($args);
    if (!is_array($array)) return false;
    foreach ($args as $n) if (!isset($array[$n])) return false;
    return true;
}
?>

Use: isset_array($array, 'key1', 'key2', 'key3', 'key4')
First parameter has the array; following parameters has the keys you want to check.

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ayyappan dot ashok at gmail dot com ¶

8 years ago


Return Values :
Returns TRUE if var exists and has value other than NULL, FALSE otherwise.

<?php
$a=NULL;
$b=FALSE; //The out put was TRUE. 
$c=TRUE;
$d='';
$e="";
if(isset($b)):
echo "TRUE";
else:
echo "FALSE";    
endif;
?>
Could any one explain me in clarity.

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mandos78 AT mail from google ¶

16 years ago


Careful with this function "ifsetfor" by soapergem, passing by reference means that if, like the example $_GET['id'], the argument is an array index, it will be created in the original array (with a null value), thus causing posible trouble with the following code. At least in PHP 5.

For example:

<?php
$a = array();
print_r($a);
ifsetor($a["unsetindex"], 'default');
print_r($a);
?>

will print 

Array
(
)
Array
(
    [unsetindex] => 
)

Any foreach or similar will be different before and after the call.

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Cuong Huy To ¶

13 years ago


1) Note that isset($var) doesn't distinguish the two cases when $var is undefined, or is null. Evidence is in the following code.



<?php

unset($undefined);

$null = null;

if (true === isset($undefined)){echo 'isset($undefined) === true'} else {echo 'isset($undefined) === false'); // 'isset($undefined) === false'

if (true === isset($null)){echo 'isset($null) === true'} else {echo 'isset($null) === false');              // 'isset($null)      === false'

?>



2) If you want to distinguish undefined variable with a defined variable with a null value, then use array_key_exist



<?php

unset($undefined);

$null = null;



if (true !== array_key_exists('undefined', get_defined_vars())) {echo '$undefined does not exist';} else {echo '$undefined exists';} // '$undefined does not exist'

if (true === array_key_exists('null', get_defined_vars())) {echo '$null exists';} else {echo '$null does not exist';}                // '$null exists'

?>

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andreasonny83 at gmail dot com ¶

9 years ago


Here is an example with multiple parameters supplied

<?php
$var = array();
$var['val1'] = 'test';
$var['val2'] = 'on';

if ( isset( $var['val1'], $var['val2'] ) && $var['val2'] === 'on' ) {
    unset( $var['val1'] );
}
print_r( $var );
?>

This will output:
Array
(
    [val2] => on
)

The following code does the same calling "isset" 2 times:

<?php
$var = array();
$var['val1'] = 'test';
$var['val2'] = 'on';

if ( isset( $var['val1'] ) && isset( $var['val2'] ) && $var['val2'] === 'on' ) {
    unset( $var['val1'] );
}
print_r( $var );
?>

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francois vespa ¶

13 years ago


Now this is how to achieve the same effect (ie, having isset() returning true even if variable has been set to null) for objects and arrays

<?php

// array

$array=array('foo'=>null);

return isset($array['foo']) || array_key_exists('foo',$array) 
  ? true : false ; // return true

return isset($array['inexistent']) || array_key_exists('inexistent',$array) 
  ? true : false ; // return false

// static class

class bar

{
  static $foo=null;
}

return isset(bar::$foo) || array_key_exists('foo',get_class_vars('bar'))
  ? true : false ; // return true

return isset(bar::$inexistent) || array_key_exists('inexistent',get_class_vars('bar'))
  ? true : false ; // return false

// object

class bar
{
    public $foo=null;
}

$bar=new bar();

return isset($bar->foo) || array_key_exists('foo',get_object_vars($bar)) 
  ? true : false ; // return true

return isset($bar->inexistent) || array_key_exists('inexistent',get_object_vars($bar)) 
  ? true : false ; // return true

// stdClass

$bar=new stdClass;
$bar->foo=null;

return isset($bar->foo) || array_key_exists('foo',get_object_vars($bar)) 
  ? true : false ; // return true

return isset($bar->inexistent) || array_key_exists('inexistent',get_object_vars($bar)) 
  ? true : false ; // return true

?>

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Andrew Penry ¶

19 years ago


The following is an example of how to test if a variable is set, whether or not it is NULL. It makes use of the fact that an unset variable will throw an E_NOTICE error, but one initialized as NULL will not. 

<?php

function var_exists($var){
    if (empty($GLOBALS['var_exists_err'])) {
        return true;
    } else {
        unset($GLOBALS['var_exists_err']);
        return false;
    }
}

function var_existsHandler($errno, $errstr, $errfile, $errline) {
   $GLOBALS['var_exists_err'] = true;
}

$l = NULL;
set_error_handler("var_existsHandler", E_NOTICE);
echo (var_exists($l)) ? "True " : "False ";
echo (var_exists($k)) ? "True " : "False ";
restore_error_handler();

?>

Outputs:
True False

The problem is, the set_error_handler and restore_error_handler calls can not be inside the function, which means you need 2 extra lines of code every time you are testing. And if you have any E_NOTICE errors caused by other code between the set_error_handler and restore_error_handler they will not be dealt with properly. One solution:

<?php

function var_exists($var){
   if (empty($GLOBALS['var_exists_err'])) {
       return true;
   } else {
       unset($GLOBALS['var_exists_err']);
       return false;
   }
}

function var_existsHandler($errno, $errstr, $errfile, $errline) {
    $filearr = file($errfile);
    if (strpos($filearr[$errline-1], 'var_exists') !== false) {
        $GLOBALS['var_exists_err'] = true;
        return true;
    } else {
        return false;
    }
}

$l = NULL;
set_error_handler("var_existsHandler", E_NOTICE);
echo (var_exists($l)) ? "True " : "False ";
echo (var_exists($k)) ? "True " : "False ";
is_null($j);
restore_error_handler();

?>

Outputs:
True False
Notice: Undefined variable: j in filename.php on line 26

This will make the handler only handle var_exists, but it adds a lot of overhead. Everytime an E_NOTICE error happens, the file it originated from will be loaded into an array.

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packard_bell_nec at hotmail dot com ¶

16 years ago


Note: isset() only checks variables as anything else will result in a parse error. In other words, the following will not work: isset(trim($name)). 

isset() is the opposite of is_null($var) , except that no warning is generated when the variable is not set.

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Ashus ¶

15 years ago


Note that array keys are case sensitive.

<?php
$ar['w'] = true;

var_dump(isset($ar['w']),
      isset($ar['W']));
?>

will report:
bool(true) bool(false)

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Anl zselgin ¶

15 years ago


Note: Because this is a language construct and not a function, it cannot be called using variable functions.

So why it is under "Variable handling Functions". Maybe there should be some good documentation field for language constructs.

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Hayley Watson ¶

7 years ago


If you regard isset() as indicating whether the given variable has a value or not, and recall that NULL is intended to indicate that a value is _absent_ (as said, somewhat awkwardly, on its manual page), then its behaviour is not at all inconsistent or confusing.

It's not just to check for uninitialised variables - a lot of the time those are just due to sloppy coding. There are other ways a variable could fail to have a value (e.g., it's meant to hold the value returned from a function call but the function didn't have a value to return) where uninitialising the variable would not be an option nor even make sense (e.g., depending on what was to be done with the returned value).

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-1

Tee Cee ¶

18 years ago


In response to 10-Feb-2006 06:02, isset($v) is in all (except possibly buggy) cases equivalent to !is_null($v). And no, it doesn't actually test if a variable is set or not by my definition "$v is set if unset($v) has no effect".

<?php
unset($c); //force $c to be unset
var_dump($a=&$c); // NULL, but this actually sets $a and $c to the 'same' NULL.
var_dump(isset($c)); // bool(false)
var_dump($a = 5); // int(5)
var_dump($c); // int(5)

unset($c);
var_dump($a=&$c); // NULL
var_dump(isset($c)); // bool(false)
unset($c);
var_dump($a = 5); // int(5)
var_dump($c); // NULL
?>

In the following example, we see an alternate method of testing if a variable is actually set or not:
<?php
var_dump(array_key_exists('c',get_defined_vars())); // false 
var_dump(isset($c));                                // also false
var_dump($c);                                       // manipulate $c a bit...
var_dump((string)$c);
var_dump(print_r($c,true));
var_dump($a=$c);
var_dump(array_key_exists('c',get_defined_vars())); // ... still false
var_dump($c = NULL);                                // this sets $c
var_dump(array_key_exists('c',get_defined_vars())); // true!
var_dump(isset($c));                                // false; isset() still says it's unset
unset($c);                                          // actually unset it
var_dump(array_key_exists('c',get_defined_vars())); // false
var_dump($a=&$c);                                           
var_dump(array_key_exists('c',get_defined_vars())); // true!
unset($c);                                          // unset it again
var_dump(&$c);                                      // &NULL
var_dump(array_key_exists('c',get_defined_vars())); // true!
?>

Obviously, null values take up space (or they wouldn't show up in get_defined_vars). Also, note that &$v sets $v to NULL if it is unset.

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-2

randallgirard at hotmail dot com ¶

18 years ago


The unexpected results of isset has been really frustrating to me. Hence, it doesn't work how you'd think it would, (as documented) a var currently in the scope with a null value will return false.



Heres a quick solution, perhaps there are better ways of going about this, but heres my solution...



<?php

function is_set( $varname, $parent=null ) {

  if ( !is_array( $parent ) && !is_object($parent) ) {

    $parent = $GLOBALS;

  }

  return array_key_exists( $varname, $parent );

}

?>



Hence, $varname should be a mixed value of var's to check for, and $parent can be an array or object, which will default to the GLOBAL scope. See the documentation of array_key_exists for further information.



This will allow to check if a var is in the current scope, object, or array... Whether it's a null, false, true, or any value. It depends on ARRAY_KEY_EXISTS for it's functionality which also works with Objects. Feel free to improve on this anyone ;D

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