声明新类时方法出现致命错误

我有一个奇怪的错误。我有DB类和Live类。Live类不是扩展DB类。我不明白为什么当我宣布新生活时会发生这种事。错误是：

Notice: Undefined property: Live::$isOff in /home/main/Database/DB.class.php on line 570
Fatal error: Call to undefined method Live::Query() in /home/main/Database/DB.class.php on line 574

这就是我在这条线上得到的：

ON DB.Class.php

public function setCharset($charset) {
    if (self::$isOff){
        return false;
    }
    self::$charset = $charset;
    self::Query("SET NAMES '".$charset."'");
    return self;
}

ON Live.Class.php

class Live
{
   protected $PaypalLogo = '';
   protected $isQA = null;
   protected $siteURL = null;
   public function __construct() {
    DB::setCharset("utf8");//Setting charset here and that is gives Fatal Error.
    if (strstr($_SERVER['SERVER_NAME'], "ba.")) {
        $this->isQA = true;
        $this->siteURL = "http://ba.live.com";
    }
    else {
        $this->isQA = false;
        $this->siteURL = "http://www.live.com/";
    }
  }

谢谢你的建议。