Add and remove querystring variables from URLs.

Querystrings.php

<?php
/*
Add and remove querystring variables from URLs.
*/
function addQuerystringVar($url, $key, $value) {
    $url = preg_replace('/(.*)(\?|&)' . $key . '=[^&]+?(&)(.*)/i', '$1$2$4', $url . '&');
    $url = substr($url, 0, -1);
    if (strpos($url, '?') === false) {
        return ($url . '?' . $key . '=' . $value);
    } else {
        return ($url . '&' . $key . '=' . $value);
    }
}

function removeQuerystringVar($url, $key) {
    $url = preg_replace('/(.*)(\?|&)' . $key . '=[^&]+?(&)(.*)/i', '$1$2$4', $url . '&');
    $url = substr($url, 0, -1);
    return ($url);
}