Update param in query string

gistfile1.php

<?php

$qs = $_SERVER['QUERY_STRING'];
$ru = $_SERVER['REQUEST_URI'];
if($qs){
    $ru = str_replace('?' . $qs, '', $ru);
    $params = array();
    parse_str($qs, $params);
    $params['page'] = '%d';
    $paramsPairs = array();
    foreach($params as $k=>$v){
        $paramsPairs[] = $k . '=' . $v;
    }
    $format = $ru . '?' . implode('&', $paramsPairs);
} else {
    $format = $ru . '?page=%d';
}