I had a similar problem and ended up using parse_url and parse_str, which as long as the URL in the parameter is correctly url encoded (which it definitely should) allows you to access both all the parameters of the actual URL, as well as the parameters of the encoded URL in the query parameter, like so:
$get_url = "http://google.com/?var=234&key=234";
$my_url = "http://localhost/test.php?id=" . urlencode($get_url);
function so_5645412_url_params($url) {
$url_comps = parse_url($url);
$query = $url_comps['query'];
$args = array();
parse_str($query, $args);
return $args;
}
$my_url_args = so_5645412_url_params($my_url); // Array ( [id] => http://google.com/?var=234&key=234 )
$get_url_args = so_5645412_url_params($my_url_args['id']); // Array ( [var] => 234, [key] => 234 )