isset -- Determine whether a variable is set


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

Note: isset() is a language construct.

Returns TRUE if var exists; FALSE otherwise.

If a variable has been unset with unset(), it will no longer be isset(). 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.

    $a = "test";
    $b = "anothertest";

    echo isset ($a); // TRUE
    echo isset ($a, $b); //TRUE

    unset ($a);
    echo isset ($a); // FALSE
    echo isset ($a, $b); //FALSE

$foo = NULL;
    print isset ($foo); // FALSE

This also work for elements in arrays:

    $a = array ('test' => 1, 'hello' => null);

    echo isset ($a['test']);  // TRUE
    echo isset ($a['foo']);   // FALSE
    echo isset ($a['hello']); // FALSE
    echo array_key_exists('hello', $a); // TRUE

See also empty(), unset(), and array_key_exists().