If you are searching for the VALUE of that url part, and you are currectly in dummy.php3, simply use $url.
If you want the string "url=hallo.html", use the substring function. Have a look at this:
$my_url_path = "dummy.php3?url=hallo.html";
$pos = strpos($my_url_path, "?"); // returns 10
$part_after_question_mark = substr($my_url_path, $pos + 1); // returns "url=hallo.html";
I hope this is what you meant.