# isset

(PHP 4, PHP 5)

isset — Determine whether a variable is set

### Description

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.

### Parameters

`var`

  The variable to be checked.

`var`

  Another variable ..

`...`

### Return Values

Returns `TRUE` if `var` exists; `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
$a = array ('test' => 1, 'hello' => NULL);
var_dump(isset($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
?>`

### 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

### See Also

- empty()
- unset()
- defined()
- the type comparison tables
- array_key_exists()
- is_null()
- the error control @ operator