Maarten Balliauw has created a set of PHP classes for creating SpreadsheetML documents. They're available under a GPL license from Maarten's blog. He has implemented the basic concepts of writing cell values and formulas and storing the result in an Open XML package.
For those who have been asking here and elsewhere about how to create Open XML documents from PHP, Maarten has demonstrated exactly how it to do it.
Thanks to Wouter for pointing out Maarten's work.