Add and remove query string values

// https://gist.github.com/philfreo/1040968


/**
 * Add the querystring variable $key with a value $value to $url.
 * If $key is already specified within $url, it will replace it.
 * 
 * Based upon http://www.addedbytes.com/code/querystring-functions/
 * 
 * @param string $url
 * @param string $key
 * @param string[optional] $value
 * @return string
 */
function add_querystring_var($url, $key, $value = '') {
    $url = preg_replace('/(.*)(\?|&)' . $key . '=[^&]+?(&)(.*)/i', '$1$2$4', $url . '&');
    $url = substr($url, 0, -1);
    
    $value = $value ? "=".urlencode($value) : '';
    
    if (strpos($url, '?') === false)
        return ($url . '?' . $key . $value);
    else
        return ($url . '&' . $key . $value);
}
/**
 * Remove the Querystring variable $key and its value from the given $url.
 * @param string $url
 * @param string $key
 * @return string
 */
function remove_querystring_var($url, $key) {
    $parts = parse_url($url);
	$qs = isset($parts['query']) ? $parts['query'] : '';
	$base = $qs ? mb_substr($url, 0, mb_strpos($url, '?')) : $url; // all of URL except QS
	
	parse_str($qs, $qsParts);
	unset($qsParts[$key]);
	$newQs = rtrim(http_build_query($qsParts), '=');
	
	if ($newQs)
		return $base.'?'.$newQs;
	else
		return $base;
}