I think it's probably easier to create unmanageable spaghetti with PHP. It's not that you can't build a large project in PHP, it's that it's easier to foul up.
By far the part of PHP that shines for small projects and causes havoc in large is the way includes work. It's possible to be disciplined about this, but it's not common. I remember many times searching in vain for some function or another that was included somewhere, somehow. Sometimes even in code that I wrote entirely!
Facebook and Wordpress and Digg being in PHP isn't a decision that was made with a large architecture in mind, it was done because the people who wrote those projects knew PHP (And in Wordpress's case, because the people who use it need it in PHP).
One of our struggles at Facebook was actually with circular includes that made our php codebase super tasty spaghetti.
File A includes File B includes File C includes File A by about 10000 files.
It was for a very scary trying to change a core library during this time because it was almost impossible to figure out where all it was included and impossible to test all the code that touched it. Additionally, we were basically loading up our entire init stack on every page load and async request because as soon as you loaded up one file all the circular dependencies would load up the entire stack. This had a big performance overhead prior to our switch to HipHop.
To this end we developed a new include system for library files that forces developers to be sane. Every module in the our library files must explicitly include everything that they need and it forbids circular dependencies. If module A requires module B, module B cannot require module A.
Making this change took a long time (in some places we are still untangling the code), but our core code is now infinitely more manageable and most importantly testable.
We have a static analyzer that runs when we submit a diff yes or try to commit, but in our "require_module" we also check for circular dependencies at run time. If you create one you get the module stack dumped into your log and promptly exited with an error message.
You can't blame PHP because the developers using it write messy code.
I'll have the same problem in Java. I'll need to find out where I implemented some logic, and have to start digging through classes and following my own breadcrumbs until I find it sometimes.
Just because I commented my code poorly, or put logic in the wrong place isn't the fault of Java, it's my own, and the languages, Java and PHP, in my opinion, are easy and fun to work with for the most part.
It happens with any language. But at least in Java you're limited (as far as I know) to looking through what you've explicitly imported. If you include a file in PHP, you have access to everything that that file included in itself, and everything that those files included, and so forth.
Excuse my ignorance, but don't you have the same behavior with `require` in Ruby? How are the behaviors of Ruby's `require` and PHP's `include_once` different?
Today not many people create large apps without a solid framework at the bottom, be it Zend, symfony, Rails, Django or whatever.
Wordpress is pretty awful but the codebase is also pretty old, rewriting everything would be loads of work and make many plugins unusable.
By far the part of PHP that shines for small projects and causes havoc in large is the way includes work. It's possible to be disciplined about this, but it's not common. I remember many times searching in vain for some function or another that was included somewhere, somehow. Sometimes even in code that I wrote entirely!
Facebook and Wordpress and Digg being in PHP isn't a decision that was made with a large architecture in mind, it was done because the people who wrote those projects knew PHP (And in Wordpress's case, because the people who use it need it in PHP).