I've run server ops (and do development) for significantly loaded RoR and PHP applications (and still do), and I can say that there is more quality information available for RoR than there is PHP for setting up high scale and stable setups.
When setting up a RoR stack, all the information I needed was available, the infrastructure was pretty stable and was easy to debug problems. PHP on the other hand, had close to no information on dealing with high load. The optcache libraries are poorly documented, nobody is really talking about it, and running anything but Apache (for performance, of course) under fcgi was horrifically problematic.
In the end we had to write our own monitoring tools to deal with all the oddities we discovered with the standard fcgi spawner (there were many), and when trying to mitigate memory leaks we discover that there are 'many known leaks in the PHP stdlib, but that's OK because everyone runs short-lived processes and restarts them regularly so we don't need to fix them'. My idea of a robust (fcgi) server would be that it doesn't require constant restarts, but that's just me.
RoR is only one framework in the vast number of Ruby libraries. I have worked in both technologies and there are more well designed, useful and current libraries and tools surrounding Ruby than there are PHP.
Quite honestly I'm sick of people apologising for it and pointing to the 'big players in web' as proof that PHP is OK. In my opinion it really needs to go away. In the years I worked with it, it taught me nothing but bad habits. I'm not the only one.
EDIT: Two Ruby libraries I used recently that were very high quality were Nokogiri (HTML/XML parsing) and Typhoeus (HTTP library wrapping libcurl). I don't recall ever seeing anything like those in PHP that had such intuitive APIs. Can anyone provide any counterpoints of better libraries in PHP that don't exist in Ruby? P.S. Wordpress is not an answer.