PHP is a brilliant language for small to medium-small sites.
Since most medium-small sites have an irrepressible tendency to grow into medium and even large sites, I'd stay away from PHP for anything other than a quick and simple site.
Ruby and Python are much more sustainable in the medium term and even the long term, and just as quick to throw applications together. Sure, they won't last forever, and eventually you may have to rewrite critical bits in another language, but in the meantime it works to release something quickly and keep improving it for a few years.
As a language for developing applications, PHP really isn't that bad. It has pretty damn good support for OOP (at least within recent releases). It lacks in concurrency support and other random things like namespaces (for which swombat advised against large-sized sites).
As a language for scaling... not too shabby. Many people who bash PHP for inability to scale really don't know jack shit about what they're talking about. Chances are, their app is slow because of all the O(n^2) loops... and they need to go take a class or two on algorithms, networking, and/or distributed systems.
I used to work with PHP a lot... eventually moved to Python simply because it was more succinct. Our teams are extremely small, and we needed a more expressive language; needed to get more done in less time. The rest speaks for itself...
We actually have moved from 100% PHP to: python for web apps, decent amount of Io & Ruby for utilities/monitoring and such, and working on a load balancer in Erlang... much more powerful development. And we really haven't run into many major stability problems...
Are you speaking in terms of traffic? Several very large sites run on PHP (Facebook comes to mind) while few run on Ruby (I think Twitter tops the list).
RoR is an architecture. Kind of ironic because I claim that Ruby, as a language, has better support for scaling in the networking/dist.systems view than PHP. Yet, I would like to claim that PHP scales better than RoR in the web app domain. Though, it takes deep knowledge of RoR internals to make a correct judgement; which, very few have.
Since most medium-small sites have an irrepressible tendency to grow into medium and even large sites, I'd stay away from PHP for anything other than a quick and simple site.
Ruby and Python are much more sustainable in the medium term and even the long term, and just as quick to throw applications together. Sure, they won't last forever, and eventually you may have to rewrite critical bits in another language, but in the meantime it works to release something quickly and keep improving it for a few years.
Daniel