Process mathematical equations in php

Question

A user is allowed to enter any mathematical equation they like (with one variable):

x + 5

1 - x/2

(x/3) * (56/13)

Answer 1

Eval is not Evil!!!!!

Yes it can stuff your system up completely if you write bad code - but recent PHP versions can parse an invalid expression without crashing the whole script. And there are many other ways of exposing your system by writing bad code.

That just leaves the possiblity of code injection attacks - which can easily be avoided by doing a preg_replace on everythnig which is not a safe character (i.e. 0....9, (, ), +, -, *, /, ^, .)