If you know it, stick with PHP...you can create your app rapidly instead of picking up the nuances of a new language/framework. While there are a lot of critici

If you know it, stick with PHP...you can create your app rapidly instead of picking up the nuances of a new language/framework. While there are a lot of criticisms of the language, the fact is there are tons of resources available if(when?) you need it. And, reiterating what some of the other posters have said, don't worry about the scaling. A well designed app in PHP or Django or Java or Rails (Twitter failed because of its flawed design...not because Rails sucks) or whatever else will scale.