PHP Manual: Determine if a variable is set and is not NULL

Description

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

Determine if a variable is set and is not NULL.

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.

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

Parameters

var: The variable to be checked.
...: Another variable ...

Return Values

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

Changelog

Version	Description
5.4.0	Checking non-numeric offsets of strings now returns `FALSE`.

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 5.4 changes how isset behaves when passed 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']));
?>

Output of the above example in PHP 5.3:

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

Output of the above example in PHP 5.4:

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.

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

isset