String to Variable converstion

Hello all, I am trying to build an IRR calculator using PHP. The number of cash flow streams I will have at any time will vary, so I am using a loop to build the formula. Then I am iterating to solve the equation and output the solution. As most of you know, I am solving the following for r: 0=SUM SERIES(CF(i)/{(1+r)^n)) I can build my formula sucessfully with the following; $form=""; while ($row=mysql_fetch_array($result)){ $form.="$CF/pow((1+'$r'),$N)"; } But it recognizes $form as a string, so when I put it in my "solution for r" loop: for($r=-1.00; $r<=1.00; $r=$r+.0001){ if($form>-.001 && $form < .001){ $IRR=round($r*100, 2); } } echo "$IRR%<br>"; BTW, I have also tried building the $form as a variable, but since I don't have a value for $r it just puts in zero and sets $form equal to the right side of the equation. Therfore, instead of inserting a formula into my "solution for r" loop, it inserts a constant for $form. I think my solution is to build the formula as a string and then insert it into my "solution for r" loop, convert it back to a real formula and then iterate for the solution. Is this possible? If not, any suggestions?