PHP Reference

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.

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.


<?php



$var = '';



// This will evaluate to TRUE so the text will be printed.

if (isset($var)) {

    echo "This var is set 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



?>

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(), and the error control @ operator.