Encrypting a message

Hi guys I really need to find a way diferent from the functions that come with php like mcrypt, cause my server do not use these. I would really apreciate some sort of exemple, since I'm not being able to crack the thing. Since I'm trying not to simply change the characters from one to another, it more dificult. It would be usefull like to have 4 variables with the alphabet and the first letter were taken from the first variable and the second from the second variable, and this foward, my problem starts when the number of variables isn't enough for all letters to be changed, so how to start the fifth letter in the first variable instead to look for a fifth variable that isn't there? Thanks, Rodrigo