One of the things that I tell in my Open XML workshops is that when you insert a new row in a spreadsheet, before rows that have formulas in them, you probably have to manually update those formulas since they now reference rows that have moved because of the insertion. To support this better you need a framework that implements a formula parser, but which also updates the formulas for you, not an easy task!
After reading Maarten's blog, I noticed that his PHP library already does this! See his 1.3.0 release.
Almost enough to make me start programming PHP Maarten!