modify_url.php

<?php
function modify_url($mod, $url = FALSE){
    // If $url wasn't passed in, use the current url
    if($url == FALSE){
        $scheme = $_SERVER['SERVER_PORT'] == 80 ? 'http' : 'https';
        $url = $scheme.'://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
    }

    // Parse the url into pieces
    $url_array = parse_url($url);

    // The original URL had a query string, modify it.
    if(!empty($url_array['query'])){
        parse_str($url_array['query'], $query_array);
        foreach ($mod as $key => $value) {
            if(!empty($query_array[$key])){
                $query_array[$key] = $value;
            }
        }
    }

    // The original URL didn't have a query string, add it.
    else{
        $query_array = $mod;
    }

    return $url_array['scheme'].'://'.$url_array['host'].'/'.$url_array['path'].'?'.http_build_query($query_array);
}
?>