@ solenoid: Your code was very helpful, but it fails when the current URL has no query string (it appends '&' instead of '?' before the query). Below is a fixed version that catches this edge case and corrects it.
<?php
function modify_url($mod) 
{ 
 $url = "http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']; 
 $query = explode("&", $_SERVER['QUERY_STRING']);
 if (!$_SERVER['QUERY_STRING']) {$queryStart = "?";} else {$queryStart = "&";}
 // modify/delete data 
 foreach($query as $q) 
 { 
 list($key, $value) = explode("=", $q); 
 if(array_key_exists($key, $mod)) 
 { 
 if($mod[$key]) 
 { 
 $url = preg_replace('/'.$key.'='.$value.'/', $key.'='.$mod[$key], $url); 
 } 
 else 
 { 
 $url = preg_replace('/&?'.$key.'='.$value.'/', '', $url); 
 } 
 } 
 } 
 // add new data 
 foreach($mod as $key => $value) 
 { 
 if($value && !preg_match('/'.$key.'=/', $url)) 
 { 
 $url .= $queryStart.$key.'='.$value; 
 } 
 } 
 return $url; 
} 
?>