PHP之如何让循环的数值相加，得出一个总和

$num  = 0;
while($row = mysql_fetch_array($query)){
  $a = $row["zhong"];
  $b = $row["shuliang"];
  $c = $a*$b;
  $num = $num + $c;//这行才是相加
  echo $c.'<br />';  
}
echo $num;