This should work for what you are trying to achieve
$string = "domain.ou/path?query=test";
$str_arr_domain = explode ("/", $string);
$str_arr_params = explode ("?", $string);
$newpath = "/api/path2/";
$final_url = $str_arr_domain[0].$newpath.$str_arr_params[1];
print_r($final_url);