If there was only one, it would be Rector. Rector is extremely powerful and brilliant and, you know what, it's free!
Rector is a tremendous application to scan and automatically upgrade your codebase to a given version of PHP and this means, too, to inspect how you're coding.
Rector will look at every line of code, every loop, every conditional test, every function, every class in a PHP source code and suggest improvements.
In this role, it will not only detect improvements to my code (like a static analysis tool) but, and this is the biggest advantage for me, it will show me how I can become a better developer.
I REALLY LOVE RECTOR.