一个生成唯一GUID的方法

function create_guid(){
    $id = strtoupper(md5(uniqid(mt_rand(),true)));
    $h = chr(45);//"_"
    $tid = chr(123)//"{"
        .substr($id,0,8).$h
        .substr($id,8,4).$h
        .substr($id,12,4).$h
        .substr($id,16,4).$h
        .substr($id,20,12)
        .chr(125);//"}"
    $tid = str_replace('{','',$tid);
    $tid = str_replace('}','',$tid);
    $tid = str_replace('-','',$tid);

    return $tid;
}

逻辑不懂，会生成一个唯一性质的ID