如何把循环中的内容逐个相加

for($counter=$i; $counter>=1; $counter--){

			$weight="weight_$counter";

			$single_price="single_price_$counter";

			$total_price="$_POST[$weight]"*"$_POST[$single_price]";

			$id="id_$counter";

		}

$big_total_price=$total_price+.......;

大概意思就是把每一次for循环中的$total_price进行相加，最后得出来一个总的$big_total_price，这样的话应该怎么写？
------解决方案--------------------
$big_total_price=0;
for($counter=$i; $counter>=1; $counter--){
            $weight="weight_$counter";
            $single_price="single_price_$counter";
            $big_total_price+="$_POST[$weight]"*"$_POST[$single_price]";
            $id="id_$counter";
        }