This look likes simple question... I view your source code in your form, and you put $last_name in the NAME field in each form...
I think you should know the different between variable and data.
The NAME should be =last_name and the value will be $last_name.
so that means your form should be like this:
input type=text name=last_name
then the output will be
Last Name = $last_name
Got it?