GUID生成函数

/**
 * GUID生成函数
 * @return string
 */
function create_guid() {
    $charid = strtoupper(md5(uniqid(mt_rand(), true)));
    $hyphen = chr(45);// "-"
    $uuid = 
     substr($charid, 0, 8).$hyphen
    .substr($charid, 8, 4).$hyphen
    .substr($charid,12, 4).$hyphen
    .substr($charid,16, 4).$hyphen
    .substr($charid,20,12);
    return $uuid;
}