I have a a field called "activation_code" in my Members table.
Once a Visitor activates his/her account - and becomes a Member - the value in this field is erased.
What is the proper way to check this field to see if it is Null?
what do you mean by 'erased'?
You can try
where activation_code is null
where activation_code is not null
$q = "UPDATE member
WHERE (email=? AND activation_code=?)