The rules() method has fixed (better say "side effects free") result -- it uses fixed values only. Sure, it calls getPhoneNumberRules() from a trait but it also returns fixed array (always the same). It does not make changes anywhere else (internal state or external storage).

The messages() method calls a method from a trait that calls __() to get translated message... that can come from a different source (a DB, a file) and therefore potentially can throw exceptions if a storage (file/DB) is not readable. The IDE does not know for sure if __() makes changes to a file/DB -- it's not annotated as pure and PhpStorm cannot make such decision from the methods that it uses.

P.S. If this inspection annoys you (which I can understand) then I suggest you just ignore such inspection and lower its severity in PhpStorm Settings to "No highlighting, only fix" (Settings (Preferences on macOS) | Editor | Inspections | PHP | Attributes | '#[Pure]' attribute can be added)

You are missing a lot of use-cases

• rand()
• database interaction
• file IO
• static variables
• calls to other functions with global
• import/require statements inside functions
• functions with internal state like ob_get_contents()
• mutating array pointers

There is probably a lot of stuff I'm not thinking of. PHP has a very stateful design.

Writing a complex application from scratch, I would definitly not use "bare PHP" : I would certainly use a framework : they provide :

• a large number of useful classes/methods
• some set of rules -- like "in which directory should controllers be saved", "how to write a view", ...
• MVC -- i.e. they help with better structuration of the project

Note that this answer is valid for both personnal projects, and professionnal projects.

There are several great Frameworks in PHP, like :

• Zend Framework
• Symfony
• CakePHP
• Code Igniter
• Kohana

One thing to remember : learning how to use a framework well will take some time : starting with a small project, before going for a big one, would probably be a good idea ;-)

Now, when you're asking yourself the question of "which framework should I choose", it's mostly a matter of personnal preferences... And here are a couple of questions/answers that might help a bit :

• PHP Framework Decision - Analysis paralysis!
• To use a PHP framework or not?
• PHP - MVC framework?
• Which PHP Framework is right for this project?
• Best PHP framework for an experienced PHP developer?

As you're asking me which Framework I would choose ; well :

• I really like Zend Framework, and often use Doctrine as ORM (it's the default ORM of Symfony, but can be used very easily with ZF)
• If I had to choose another one, I would probably go with Symfony, as I've seen it used on a couple of projects at work, and know many people who work with it and like it