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isset

(PHP 3, PHP 4 )

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.

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)) {
    print "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.

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