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(PHP 3, PHP 4, PHP 5 )

isset -- Determine whether a variable is set

Description

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

Returns TRUE if var exists; FALSE otherwise.

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.

<?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);

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

?>

Related Function(s)

unset()

defined()

empty()

array_key_exists()