_setURL(‘http://mydomain.com?gender=male’, ‘site’, ‘back’);
function _setURL($url, $key, $value) {
    $separator = (parse_url($url, PHP_URL_QUERY) == NULL) ? '?' : '&';

    $query = $key."=".$value;
    $url .= $separator . $query;
    var_dump($url); exit;
}

$frontLink = (isset($_GET['gender'])) ? 'mydomain.com?gender='.$_GET['gender'].'&site=front':'mydomain.com?site=front';
$backLink = (isset($_GET['gender'])) ? 'mydomain.com?gender='.$_GET['gender'].'&site=back':'mydomain.com?site=back';

$maleLink = (isset($_GET['site'])) ? 'mydomain.com?gender=male&site='.$_GET['site']:'mydomain.com?gender=male';
$femaleLink = (isset($_GET['site'])) ? 'mydomain.com?gender=female&site='.$_GET['site']:'mydomain.com?gender=female';

function add_get_parameter($arg, $value)
{
    $_GET[$arg] = $value;

    return "?" . http_build_query($_GET);
}

<?
function requestUriAddGetParams(array $params)
{
    $parseRes=parse_url($_REQUEST['REQUEST_URI']);
    $params=array_merge($_GET, $params);
    return $parseRes['path'].'?'.http_build_query($params);
}