Often, the best tool in web software is the one you can get the most/cheapest labor for so you can actually get your product completed and on the market.
No one cares if your software is programed in the latest tech with the very best techniques - all they care about is if your business is viable. PHP fills this niche excellently.
As a developer I make my living off PHP. I'd rather play with nicer languages, but frankly that isn't where the money is where I am.