PHP is a template language at heart, so it's a little disingenuous to say that it doesn't force you to learn one, as it does - it's like the erb template language part of rails without the controllers and models. That's fine for what it is, but becomes cumbersome if you want to persist data to the db, handle forms for your models, etc at which point you'd often be better served looking at a framework which does a lot of that work for you. Some are available in PHP, but there are IMHO better options in other languages, and of course, better languages.

Do we need to have one language which scales from 'throw together a web page' to complex web apps? Is it even possible or desirable?

While I think it'd be nice to see Python and Ruby have the number of users that PHP does (though perhaps it wouldn't, with scale comes a worse NSR), it's probably not going to happen, simply because they provide a bit more structure, and many people don't want structure. Those people will be happy with PHP (until they're not, and realise they needed that structure after all), and I don't think Ruby or Python frameworks really have any ambition of catering to them. That's fine and is not a failing on either side. For example no one on the ruby side is going to stop using gems as package management because users starting out might be confused, as they are just too useful. You will find plenty of web frameworks for ruby which are simply one gem to install and off you go though - hardly onerous.

It would certainly be possible to write a better PHP, but all the people who might do that are busy using slightly more complex frameworks which make web development easier.