great stuff

switch ($currentop) {
case "-":
$airemax = $airemax - $calc2;
$airemin = $airemin - $calc1;
break;
default:
$airemax += $calc2;
$airemin += $calc1;
}

You can use the += for minus too you know :) Just use -=
*= and /= also works :)

I like how you strip the functionstring and make it actual mathlike. Great stuff! :)
Cheers

It's very "basic" at the moment, I'm afraid. I'm still working on most of the stuff - and yes, I perfectly know += and -=. Used them once, but heh. Both are equally fast.

As for the mathlike, try finding another way to make the bloody computer, whose mind works in decimal, understand how to parse an equation. It's bloody hard. I did what I could, and the next time I send an update, it'll be better (Hopefully), and able to differentiate trig functions (and logarithms). It just takes time to build an intelligence, however simple it is.