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Determine whether a variable is set (PHP 4, PHP 5)
bool isset ( mixed var [, mixed var [, ...]] )

Determine whether a variable is set.

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.



The variable to be checked.


Another variable ..


Return Values

Returns TRUE if var exists; FALSE otherwise.


Example 2596. isset() Examples


= '';

// This will evaluate to TRUE so the text will be printed.
if (isset($var)) {
"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:


= 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




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.


Because this is a language construct and not a function, it cannot be called using variable functions