Add $String to $Num ???

Hi, I have a variable called $String which can contain 3 random numbers like so... $String="12,94,423"; I then have $Num which is a single number from 1 - 20, I want to add $String and $Num together like this $String$Num So I would end up with $String1="12,94,423"; and then I can enter that value into my session.... session_register(String.$Num); I have tried everyway I can think of but it never works The reason for doing this is so I can store multiple $Strings in the session and use them later for a search. Can anyone help? Thanks Ade