Danne Lundqvist has posted about the process he went through when developing a solution to create Excel files in PHP:

I'm on this project at work where we need to create an excel file and send it via email or ftp to a recipient. The normal answer to this is to create a comma separated file and name it .xls and excel will work it out. However, that only works if you open it directly through an http request and are able to set the http headers correctly.

To get around these limitations, he found two solutions (thanks Google!) in PEAR's Spreadsheet_Excel_Writer and an MS-Excel Stream Handler class. He includes code that gives an example of another solution, one that packs data into a binary string and returns it as a part of the Excel document.