Creating protected Excel files from PHP

Hi, I would like to know if any from you guys have a solution to my problem. I use PHP and a PEAR class (PEAR::Spreadsheet_Excel_Writer) to generate some reports as Excel files. My problem is that I want these files to be protected with a password (because these reports will be downloaded to the user's computer, and they must be unaccessible to someone else). I know you can set up a password for an excel file, but I don't know if you can accomplish this with PHP. If you know how to do this (or if this can be done) please let me know. Thanks!