Dynamic logical expression parsing/evaluation in PHP?

Question

I have a need to evaluate user-defined logical expressions of arbitrary complexity on some PHP pages. Assuming that form fields are the primary variables, it would need to:

Answer 1

If I understand the problem correctly, you want the users to write out functions in non-PHP, and then have PHP interpret it?

If so, you could simply take their string and replace "lt" with "<" and "gt" with ">" ... then do eval().

I have a hunch the problem isn't this simple, but if it is, eval() could do the job. Of course, then you're opening yourself up for any kind of attack.