Hadoop LongSumReducer(Hadoop LongSumReducer)

 通过打印检查你的两个foreach变量在该函数是数组，我的意思是你的$ tax得到了行和$ tax得到了正确的序列化数据，你可以通过使用is_array（）嵌入if条件来忽略该错误，这里是一个演示.. 
 
if (is_array($taxes)) {
    foreach ($taxes as $tax) {
        $tax = unserialize($tax);
       if(is_array($tax)){    
         foreach ($tax as $id => $tax_data) {
            $subtotal += !empty($tax_data['tax_subtotal']) ? $tax_data['tax_subtotal'] : 0;
         }
       }
    }
}

check out by print your two foreach variable in that function is array, I mean your $taxes got rows and $tax got proper serialize data , you can ignore that error by embed an if condition with is_array(), here is a demonstration .. 
 
if (is_array($taxes)) {
    foreach ($taxes as $tax) {
        $tax = unserialize($tax);
       if(is_array($tax)){    
         foreach ($tax as $id => $tax_data) {
            $subtotal += !empty($tax_data['tax_subtotal']) ? $tax_data['tax_subtotal'] : 0;
         }
       }
    }
}