http://www.yoursite.com/offers/838?&SITEID=2172

http://www.yoursite.com/offers/838

$str = 'http://www.yoursite.com/offers/838?&SITEID=2172';

function remove_query_arg($var, $url = NULL){
    if(!$url){
        $url = $_SERVER['REQUEST_URI'];
    }
    $parsed_url = parse_url($url);

    $query_vars = explode('&', $parsed_url['query']);


    foreach($query_vars as $key => $value){
        $query_vars[$key] = explode('=', $query_vars[$key]);
        $query_variables[$query_vars[$key][0]] = $query_vars[$key][1];
    }

    if(is_array($var)){
        foreach($var as $value){
            unset($query_variables[$value]);
        }
    }
    elseif(is_string($var)){
        unset($query_variables[$var]);
    }

    $query_vars = array();

    foreach($query_variables as $key => $value){
        $query_vars[] = $key.($value !== NULL || !empty($value) ? '='.$value : '');
    }
    $query_str = '';
    $query_str = implode('&',$query_vars);

    return (isset($parsed_url['scheme']) && !empty($parsed_url['scheme']) ? $parsed_url['scheme'].'://' : '').$parsed_url['host'].(isset($parsed_url['path']) && !empty($parsed_url['path']) ? $parsed_url['path'] : '').(!empty($query_str) ? '?'.$query_str : '');
}

echo remove_query_arg('SITEID', $str);

$url = "http://www.yoursite.com/offers/838?&SITEID=2172";
$parts = parse_url($url);

$url = $parts["scheme"] . "://" . $parts["host"] . $parts["path"];

$url=http://www.yoursite.com/offers/838?&SITEID=2172
$result=explode('?',$url)
print_r($result);

array
{
 [0]=>http://www.yoursite.com/offers/838
 [1]=>?&SITEID=2172
}

$url = "http://www.yoursite.com/offers/838?&SITEID=2172";
list($path, $query) = explode("?", $url, "2");
var_dump($path);

string 'http://www.yoursite.com/offers/838' (length=34)

$url = "http://www.yoursite.com/offers/838?&SITEID=2172";
$str = substr($url, strpos($url, 0, "?&SITEID"));
// $str results in "http://www.yoursite.com/offers/838"

^(.+?)(?&SITEID|$)

$1

&?SITEID=[^&s]+