This is the same advantage as Ruby - the community for both languages has been more vibrant and interesting than PHP for years.
PHP is a mature language and isn't trendy, but it is far more vibrant, broad, and interesting than Ruby or Python.
apt-cache depends a* | grep -c python: 18711
apt-cache depends a* | grep -c php: 1596
apt-cache depends a* | grep -c ruby: 2839
Not that it proves anything but it might be an indicator.
Security updates routinely break functionality with no warning or knowledge on core and modules, and the community is architected to make it far more difficult to contribute new modules than necessary (not to mention submitting patches). I won't even update the Date module - ever (http://drupal.org/project/date); I can't count the number of times an update has introduced more bugs than it fixed. It's irresponsible.
PHP is a great language, and there are a lot of great tools, frameworks and websites written with it, but it doesn't mean it's the only game in town. What is most important is using the right tool for job, whatever it may be. Sometimes it's Drupal/Wordpress, sometimes it's Rails. Github's language page illustrates that pretty well: https://github.com/languages
Size doesn't matter. Utility does. Lots of good languages exist, but they're all just tools to be used by the programmer. I use Ruby because the community is rapidly developing fantastic, useful gems and tools that meet my needs and make my life as a developer easier. PHP does not meet my needs in the same way. Thankfully, the existence and usefulness of one language/platform does not negate the existence and usefulness of many others.
First, PHP is a great language. It definitely has its strengths. It was the second language that I ever used to create web sites (right after Perl).
With that said, the number of people using something does not inherently make it superior to all of the other choices. PHP, Python, Ruby, etc all have their proponents and they all have their valid use cases. Which one someone chooses to use is largely a matter of personal preference.
Drupal is a great package, but I found it to be cumbersome and unwieldy for my purposes, which is why I started fiddling around with Django. Choices are good - HipHop, Wordpress, etc do not innovate in a vacuum.
Innovation is facilitated by maintaining an open mind and looking to offer improvements and variations on existing solutions. There is no war between the languages. Everyone can learn from each other - and they often do.
Now, as to maturity. Both PHP and Ruby have been around since roughly 1995. (People sometimes mistakenly believe that Ruby did not exist until Ruby on Rails, but it was around for quite a bit of time before that.) Python has actually been around (publicly) since 1991. All three languages are mature by any definition of that word.
As to broadness, I think that it is worth noting that Python has really come into its own in the area of scientific and mathematic applications. I have always found Ruby to be an amazing general purpose text processing and scripting language (in addition to web programming). PHP, on the other hand, I have not actually seen in extremely heavy usage outside of the web environment. I would be pleased to know of any though, but you must admit that the usage of PHP in developing web sites fairly drowns out its usage in more general cases.
As to community size, I believe that quality not quantity is what counts. Both Python and Ruby have a lot of bright people working on a lot of really great projects. Of course, PHP does as well, but there is no reason to be outright dismissive of alternatives (you know, Perl is actually pretty neat too, but I rarely use it nowadays).
Additionally, where do you have numbers to support your assertion that Ruby is in decline? That is a rather strong statement - you should have numbers to back it up, and TIOBE is not a very good source of data for this.