php stdClass检查属性是否存在

我在这里已经阅读了一些类似的问题但不幸的是找到了我的案例的解决方案.

API连接的输出的一部分是;

stdClass Object ( [page] => 0 [items] => 5 [total] => 5 [saleItems] => stdClass Object ( [saleItem] => Array ( [0] => stdClass Object ( [reviewState] => approved [trackingDate] => 2013-11-04T09:51:13.420+01:00 [modifiedDate] => 2013-12-03T15:06:39.240+01:00 [clickDate] => 2013-11-04T09:06:19.403+01:00 [adspace] => stdClass Object ( [_] => xxxxx [id] => 1849681 ) [admedium] => stdClass Object ( [_] => Version 3 [id] => 721152 ) [program] => stdClass Object ( [_] => yyyy [id] => 10853 ) [clickId] => 1832355435760747520 [clickInId] => 0 [amount] => 48.31 [commission] => 7.25 [currency] => USD [gpps] => stdClass Object ( [gpp] => Array ( [0] => stdClass Object ( [_] => 7-75 [id] => z0 ) ) ) [trackingCategory] => stdClass Object ( [_] => rers [id] => 68722 ) [id] => 86erereress-a9e4-4226-8417-a46b4c9fd5df )

某些字符串不包含gpps属性.

我所做的如下

foreach($sales->saleItems->saleItem as $sale)
{
    $status     = $sale->reviewState;

    if(property_exists($sale, gpps)) 
    {
        $subId      = $sale->gpps->gpp[0]->_;
    }else{
        $subId      = "0-0";
    }
}

我想要的是我的gpps属性不包含在db中存储为0-0的字符串$ subId中,否则从字符串中获取数据.但是如果没有gpps,它就无法获得.

我的错误在哪里？