Is there limitation for switch case: argument's value?

Hi, I encounter a funny limitation here with switch case as below: The value for $sum is worked as expected for 1 to 8, but not for 0. When the $sum=0, the first case will be return, which is "sum=8". Is there any limitation / rules for switch case? Thanks for advice! Keith $sum=0; switch($sum) { case ($sum==8): echo "sum=8"; break;

       case ($sum==7 || $sum==6):
       echo "sum=7 or 6";
       break;

case ($sum==2 || $sum==1): echo "sum=2 or 1"; break;

       case 0:
       echo "sum=0";
       break;

default: echo "sum=3/4/5"; break; }