[ticket/12723] Add Sniff ensuring PHP files use the correct file header

PHPBB3-12723
Merge pull request #2630 from Nicofuma/ticket/12723

[ticket/12723] Add Sniff ensuring PHP files use the correct file header

* Nicofuma/ticket/12723:
  [ticket/12723] Add Sniff ensuring PHP files use the correct file header
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12723] Add Sniff ensuring PHP files use the correct file header