Variable handling Functions
PHP Manual

isset

Determine if a variable is declared and is different than null

Description

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

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