Using Excel in a PHP controlled environment

Hi, I am trying to create a web app using the Excel COM object and PHP. I'm not sure if this is the right approach or not. What I'm trying to do is to have a user log in, if they have read only access to an Excel Spreadsheet, the spreadsheet will open on the server with read only access. If they have read/write access the spreadsheet will open on the server and allow them to write to the file and save it replacing the original file. In either case if the file is open no other users should be able to access the file. This is a highly trusted workgroup environment so I'm not concerned about inadvertant overwriting etc. How would I go about creating a web app that does this? Is there already something like this available? ...or is this even possible? Thanks, Ron