PHPDoc + Some refactorisation

3 commits: 

[gitignore] Add .idea/ to .gitignore (for PhpStorm IDE). 
=> Prevent PhpStorm users to commit their .idea/ folder by error

[improvement] Replace "return;" by "return null;" (more verbose even if it's the same returned value + when I do local testing between "return;" and "return null;", I noticed a very little gain: some milliseconds)
=> I think the more verbose is the most understandable

[phpdoc] fix phpdoc and reformat some code
=> Add throws to PHPDoc and reformat some code with php-cs-fixer

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Run phpunit after and I obtain the same message than before my changes (no breaks I hope):
```
OK, but incomplete, skipped, or risky tests!
Tests: 1904, Assertions: 4283, Risky: 17.
```