[For every X, replace it with Y]

Hi,

 

I'm pretty new to PHP, and I'm trying to write my own encoding/decoding script (not with base_64).

 

I'd like to replace like all A's with B's, all B's with C's, C's with D's, and so on. How would I do this? Here's an example:

 

String #1: "ABCDEFG"

string #1 goes through my encryption

string #2 (output): "BCDEFGH".

 

How would I go about doing this? Thanks.