PHP can die when there's another language that's as dead simple to run as PHP. A new programmer can open up a text file with a php extension on (pretty much) any web server, type "<? echo "Hello World"; ?>", and it works.
Ruby is very easy to run: puts "Hello World"
As is perl: print "Hello World";
Php? <?php echo "Hello World"; ?> (wtf is all that crap, says the new programmer)
The point the OP is trying to make is the "stupid simple with the web" side of the equation. Of course the PHP side isn't as simple as "the language runs on the web". PHP runs on Apache because of the tons of work put into mod_php to integrated it and make it work as easy as possible.
You can't "just run" php on nginx, lighty, etc. Then you need to setup FastCGI instances, and now you're no different from Rails land with mongrels or thins.
apt-get install php5
Usually you don't even need that.
You are confusing Ruby for Ruby on Rails.
Ruby is easy to run. Ruby on Rails, not as much(relatively).
When people just want something nice and simple, PHP works beautifully. There's nothing wrong with the parts of the language that people who don't know programming are using to glue pieces of their website together. It's really only once you start using it for more than that when you start hating the terrible design choices and language constructs.
Leave PHP as the ubiquitous solution for simple glue, it does it quite well. When people want to do web development beyond the glue they can turn to any other number of great languages/frameworks.
That's actually a good example for why it should die (embedding code in html).