Proper way to check for string

// http://www.zachstronaut.com/posts/2009/02/09/careful-with-php-empty.html

// $mystring is NOT defined
//$mystring = null;
if (empty($mystring)) {
    // the condition passes without a warning
}

if ($mystring == null) {
    // the condition passes WITH a warning
}

if (!isset($mystring) || $mystring == null) {
    // the condition passes without a warning
    //
    // this conditional is a suitable replacement for empty() when
    // dealing with strings
}