Programming Forums - appending an excel file

I am pretty new to PHP but I feel like I've made some pretty good progress recently. I am currently trying to figure out if it is possible to pull data from a form and append it to an existing excel file that is stored on the web server. I have seen quite a bit about extensions that allow you to read from spreadsheets, but I haven't seen anything that talks about writing to a file without the use of a spreadsheet-like interface. I don't want users to be able to edit or even view the data, so my programs interaction with excel needs to be behind the scenes. I know I could use fputcsv to write to a csv file and then open that file in excel, but I was curious if there was something out there specifically for interaction with xls files. If someone could clue me in and if possible give me a short example, or a source where I might find some examples of this interaction I would really appreciate it. Thanks