function parser

Hi guys If I want to input a function (for example x = 2y + 5) and then have it stored in a database, so next time when I input the values for x and y, I would like php to retrieve the function and compute it. I know how to store and retrieve, anyone got any ideas about writing a parser to parse the function? I've wrote parsers under lisp and other languages before just not PHP. If someone wrote something like this or know of something like this, I'd rather not have to reinvent the wheel. Thanks John