iOS safe cookie session start

gistfile1.php

function cookiesafe_session_start(){
    $sn = session_name();
    if (isset($_COOKIE[$sn])) {
        $sessid = $_COOKIE[$sn];
    } else if (isset($_GET[$sn])) {
        $sessid = $_GET[$sn];
    } else {
        session_start();
        return false;
    }

    if (!preg_match('/^[a-zA-Z0-9,\-]{22,40}$/', $sessid)) {
        $newSessId = md5(microtime());
        session_id($newSessId);
        if(isset($_COOKIE[$sn]))setcookie($sn,$newSessId);
    }
    session_start();
    return true;
}