Querystring variables (See related posts)

// Add Querystring Variable
// A PHP function that will add the querystring variable $key with a 
// value $value to $url. If $key is already specified within $url, 
// it will replace it.

function add_querystring_var($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);
}
}

// Remove Querystring Variable
// A PHP function that will remove the variable $key and its value 
// from the given $url.

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