For me, and talking about large or even huge projects, it (primarily) boils down to one word: Dependencies.
The problem with a scripting language is like in every thing in the world: The greatest advantage is the greatest disadvantage at the same time.
The greatest advantage is to code free and fast. Just write a script and it will server it's purpose. No verbosity needed, simply code.
The greatest disadvantage is, in a way, to check if this script is not disturbing other scripts. Or better: Change an old script others are relying on. Are you sure that all dependencies do work out as you desired?
This is not true for "normal" web page generation, whatever normal means here. But we have a product relying on some 500k lines of source code, with customizations for clients consisting of an additional 100k lines of code, too. And I am deadly glad that a compiler checks all dependencies and warns/errors me in case I did something wrong (like, speaking lowest level here, misstyping a variable or method call).
I think this and the fact that other languages provide more simple-to-use "enterprisy" features by their nature (i.e. application servers for "bank usages") boils it down why many do not see PHP in large (or better: huge) projects.
