Just like with any language, I don't think you need to "build" one single project, it's more like having enough everyday hands-on experience to be able to choose the right way to implement the logic in the given language. Knowing how to properly use all major classes/functions, being also aware of the less known functions (in php code you'll often see people reimplementing the functionality that's already supported directly in the language, especially when working with arrays and strings), knowing all important gotchas and naturally structuring your code to avoid them... that is what I consider mastering a language...