The manual dictates that for comparison of equality you need to use ==. what you are doing is assigning a blank value to the variable by using the assignment operator =.
So your code would be
if($pw != $re-pw)
print "error: passwords do not match";
else if($username == "" || $pw == "" ||...
I have posted my product class below where I am thinking I need to add in the DiscountPrice attribute and the code for my product manager controller which contains all of the basic index, create, edit, delete.... I have also created a separate Members Only Controller which is where I am looking...