PHP is used for blogs, and one - person operations/businesses. As well as one-person managed sites - yes, like Facebook (Their controller/server management software, is a different matter)

No serious business (read more than two employees) uses PHP.

And yes, 80% of the websites, etc., are based upon PHP - see my first statement. It is good for a single person at a time, wanting to update/edit static content (like FaceBook)

But probably 99.9% of actual web commerce is done on non-PHP sites...

In short, it has serious scaleability issues that are inherent in its' design.

And yeah I have been involved in building businesses, and when it comes to developing your web presence, that is the story...

Oh, and please point out where it has anything to do with University of Waterloo?

https://en.wikipedia.org/wiki/PHP