One of the new (planned) features of PHPExcel is to implement parsing and calculating Excel formulas. One thing every developer should do is not to try to reinvent the wheel. Therefore, a Google search learned me someone wrote a Excel expression parser in JavaScript, which parses an expression into a tree.
Parsing Excel formulas (expressions) in JavaScript is done here. Someone ported this to C# too, and as of today, it is ported to PHP5 too 8-).
The only thing left to do is building this into PHPExcel, and performing calculations using the parsed tree...