I would say no because there are better languages today. Python, Ruby and C# have much nicer namespacing and development tools than php.
Your real problem however will not be the language. It will be the database layer, since this will be a lot more centralized. You can use memcached to reduce the load on the database. If you're using an ORM check that it has all the ability to run straight queries and run bulk inserts or use joins rather than lazy loaded queries to get related objects.
You should also use queues to speed page loads up. If you have slow operations that can be done outside of the page requests then push them into a queue and process them later, possibly on a different server. Emails and image resizing is a good example.
If you want a truely scalable web application you will have to step outside the Apache, MySQL bubble and start using tools like Nginx and non-relational databases.
For now, what it is best to do, is not worry too much about the problem before it even exists. Just make sure whatever framework you choose is loosely coupled so you can do things like switching out the database layer. Avoid codeigniter like the plague, it isn't modular at all and has no ORM and a really poor form library. I would recommend Django(python), there are a number of companies which do cloud hosting for it. I've seen a shop switch from PHP to Python and they ended up liking it a lot. Don't think you have to be locked into PHP, especially when you are choosing the hosting.
Edit, reply to Cartoonle
Django. But if you really want PHP, maybe Symfony, but it is known for being slow. Kohana is like CI, but more community driven and OOP, so some of the major flaws of codeigniter have been solved long ago in kohana. I've also heard nice things about Cake, but never tasted it myself. As for codeigniter, yes many like it, it is simple, but lacks a lot of good software patterns in favor of ugly shortcuts. I worked with it for a year, after a while everyone on the team agreed we were re-inventing the wheel too much and needed something else. When you've worked with it for a while the problems start to bubble up. We debated for a while, eventually ended up with Django actually. Was a hard decision but paid off with higher productivity. Plus Google use it, I think that says a lot.
Seriously python is one of the only languages used in comp sci 101 courses and professionally. One of the goals of python was to make teaching comp sci easy. It's not that scary, open up a terminal, type in python
and have a play around. There are a lot of nice tools around for it too, like pip, which is like PHP-pear but also works with git, gzip, hg and a number of other formats besides the wonderful python packages repository.
There is also a heap of packages around for it, which I really don't see nearly as much with PHP. http://djangopackages.com/
At least give it a chance by doing the tutorial, which takes ~2 hours. It comes with a built in web server so there is nothing to setup besides python itself.
http://docs.djangoproject.com/en/1.3/intro/overview/