> What I really need some with was understanding why $categoryvalue
> would
> be empty.

Try echoing out $tempbox and see if it looks as expected. Ensure that a
variable exists with the name of what it choes out. Another thing you could
do is ensure you have all errors being reported (error_reporting(E_ALL).
If $tempbox is not outputting what is expected then I would expect an error
message stating an undefined variable such as:-

Notice: Undefined variable: b in php-file on line 4

This indicates that variable b has not been defined yet I am trying to use
it.