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isset

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

issetDetermine if a variable is declared and is different than null

Description

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

Determine if a variable is considered set, this means if a variable is declared and is different than null.

If a variable has been unset with the unset() function, it is no longer considered to be set.

isset() will return false when checking a variable that has been assigned to null. Also note that a null character ("\0") is not equivalent to the PHP null constant.

If multiple parameters are supplied then isset() will return true only if all of the parameters are considered set. Evaluation goes from left to right and stops as soon as an unset variable is encountered.

Parameters

var

The variable to be checked.

vars

Further variables.

Return Values

Returns true if var exists and has any value other than null. false otherwise.

Examples

Example #1 isset() Examples

<?php

$var 
'';

// This will evaluate to TRUE so the text will be printed.
if (isset($var)) {
    echo 
"This var is 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'pie' => array('a' => 'apple'));

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

// Checking deeper array values
var_dump(isset($a['pie']['a']));        // TRUE
var_dump(isset($a['pie']['b']));        // FALSE
var_dump(isset($a['cake']['a']['b']));  // FALSE

?>

Example #2 isset() on String Offsets

<?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']));
?>

The above example will output:

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

Notes

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.

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

Note:

When using isset() on inaccessible object properties, the __isset() overloading method will be called, if declared.

See Also

add a note

User Contributed Notes 18 notes

up
94
p_ignorethis_lbowers at gmail dot com
6 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.
up
33
kurdtpage at gmail dot com
5 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
up
42
a dot schaffhirt at sedna-soft dot de
14 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
?>
up
29
beuc at beuc dot net
15 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.
up
29
yaogzhan at gmail dot com
17 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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9
ayyappan dot ashok at gmail dot com
6 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.
up
20
mandos78 AT mail from google
14 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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17
Cuong Huy To
11 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'
?>
up
18
soywiz at php dot net
16 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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8
andreasonny83 at gmail dot com
7 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 );
?>
up
9
francois vespa
11 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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11
Andrew Penry
17 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.
up
1
Hayley Watson
5 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).
up
5
Anl zselgin
13 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.
up
5
packard_bell_nec at hotmail dot com
14 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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4
Ashus
13 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)
up
0
randallgirard at hotmail dot com
16 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
up
0
Tee Cee
16 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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