I am building a PHP application where the user upload Powerpoint files. I want the other users to view it online instead of downloading. (Using a third party apps like google docs might be a little cumbersome for the users) Is it possible to write a powerpoint viewer code in PHP?
"Is it possible to write a powerpoint viewer code in PHP?"
Yes. Unfortunately, if you are asking this question, you probably won't be able to do it yourself.
If you want to try it anyways, here's a good place to start: http://msdn.microsoft.com/en-us/library/cc313106(office.12).aspx
Or you can look for a library that does that. They are probably out there, just Google it.
EDIT: Found one here: http://phppowerpoint.codeplex.com/
You want to re-write Powerpoint in PHP? I'm gonna say... very difficult at best. There are, however, tools out there that make your life easier. Also, there's a "Save as web page" option in Powerpoint, so maybe you could have your uploaders save the powerpoint as a web page, and upload that output, which I would imagine would be pretty easy for you to subsequently put up on the web.
Alternatively, if you're feeling more ambitious, you could read up on the Google docs APIs, and possibly create a portal to upload to Google docs for the contributors, and view Google docs for your visitors. Your PHP frontend could leverage the power of Google docs, but eliminate the cumbersomeness (I'm kind of surprised that's a real word).
I suspect it would be much easier to reduce the powerpoint slides to .PNG files, and build a simple PHP script to cycle through the images.