self modifying code

[Anon]

perhaps i'm in over my head, but hopefully someone php savy can pull me out by my hair.
I've been messing around with image creation using php and was coding a simple graphing calculator. I have a form set up to take an equation from the user( example: y=x x). I then convert equation variables to php variable ( x to $x ) then insert it into a y=$converted_script_entered_by_user;. But since it is only compiled once the newly converted variables that the user entered are set into the equation as just variables, not the integers they were meant to represent. the equation ends up being y=$x $x equating to 0.
help!
thanks,
josh

[Anon]

Self modifying code? From stuff the user enters? Watch PHP crash and burn.

Yes, you could use eval() to execute the string as code, but by the time you've checked it out safely enough you might as well just parse it yourself. A recursive-descent parser wouldn't be that hard to write in PHP, and is certainly adequate for simple mathematical formulae.