how to define variables as numeric

Hi, I have some issues with my code. It's to refresh account payfile at end of month. Comments are noted
in code (//).
#1) how to define variables as numeric(I've tried several examples(forums and manuals)
#2) how to add several variables together so they are known as numeric(they are all decimal 8,2 in the database)
#3) Please, someone explain the "Undefined variable: mysql".

the code:
how to define variables as numeric