生成UUID

/**
 * 生成uuid
 */
function UUId()  
{  
    $chars = md5(uniqid(mt_rand(), true));
    $uuid = substr($chars, 0, 8) . '-'
            . substr($chars, 8, 4) . '-' 
            . substr($chars, 12, 4) . '-'
            . substr($chars, 16, 4) . '-'
            . substr($chars, 20, 12);
    return $uuid;
}