Does isset () function do in php?

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

isset — Determine if a variable is declared and is different than null

Description

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

The variable to be checked.

Further variables.

Return Values

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

Examples

Example #1 isset() Examples

$var

This also work for elements in arrays:

$a

Example #2 isset() on String Offsets

$expected_array_got_string = 'somestring';
var_dump(isset($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

• empty() - Determine whether a variable is empty
• __isset()
• unset() - Unset a given variable
• defined() - Checks whether a given named constant exists
• the type comparison tables
• array_key_exists() - Checks if the given key or index exists in the array
• is_null() - Finds whether a variable is null
• the error control @ operator

p_ignorethis_lbowers at gmail dot com ¶

5 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.

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:

// 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

a dot schaffhirt at sedna-soft dot de ¶

13 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
    $abc = (object) array("def" => 123);
    var_dump(isset($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
?>

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

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

yaogzhan at gmail dot com ¶

17 years ago

If you have

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

    function

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.

$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.

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:

$a = array();
print_r($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.

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.

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

unset($undefined);
$null = null;

if (

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:

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.

andreasonny83 at gmail dot com ¶

7 years ago

Here is an example with multiple parameters supplied

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

if ( isset(

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

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

return isset(

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.

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

function

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).

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.

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.

Ashus ¶

13 years ago

Note that array keys are case sensitive.

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

will report:
bool(true) bool(false)

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...

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

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".

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:
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.