Interesting post by the cdbaby dude about how he tried to convert the entire site to Rails then scrapped the entire effort after 2 years and went back to PHP.

http://www.oreillynet.com/ruby/blog/2007/09/7_reasons_i_switched_back_to_p_1.html

I have to say one of the top 2-3 decisions I made regarding HelpSpot was using PHP. It's flexible, fast, ultra reliable, runs everywhere, is already running most places and just great software. And that's with HelpSpots dependencies on some less that common extensions like IMAP (mail handling) and mbstring for character conversions (technically this is optional in HelpSpot, but useful).