Those are what its advantages were. Now its main advantages seem to be: (1) Omnipresence and (2) Easy to migrate from a static site (I missed that one in my prior comment, and it's an important one - thanks!). All the others are relative to the environment of its birth, rather than today's environment.
Today we have choices other than Perl, C, and PHP - and many of those choices are free and Free and have great docs and large healthy communities and have languages and libraries with a better design. When people get good enough at programming to understand why global variables are bad, they should generally not be choosing PHP for new projects unless they need one of the above two features.
I don't know where this idea of heroku having poor uptime is from. Heroku has excellent uptime - even during the ec2 outage most sites had very little actual downtime.
I wrote out a list of PHP's main advantages a while ago (http://news.ycombinator.com/item?id=3347956), but I don't think there's just one.