Bump required PHP version to 7.2

[borekb]

When working on #1389, I realized that we only run tests on PHP 7.2 which made me think whether we shouldn't bump the required version anyway. Some points around this:

• PHP 5.6, VersionPress' current minimum required version (Bump required PHP version to 5.6+ #726, docs), has been unsupported for almost two years now:
  
  PHP 7.2 is the lowest fully supported version as of today.
• The performance of PHP 7 is much better.
• We could use language features like type hints.
• PHPUnit has a policy of only running on the currently supported PHP versions, also tools like PHPStan require recent PHP releases to run.

A counter-argument is that PHP 5.6 is still massively popular. Incidentally, WordPress just announced that WP 5.2 will bump the minimum required version to PHP 5.6 and a chart in that blog post shows that PHP 5.6 is used on 32% of WordPress sites – in fact, it's the most popular version out there, followed by PHP 7.0 with 18%. PHP 7.2 is third with 14%:

Still, in my eyes, it's more important to make VersionPress work reliably than to worry about legacy PHP versions. In practice, VersionPress can be run only on servers that the user has a good control of (Git is required, etc.) so I think requiring supported PHP version is not that big of an ask.

[borekb]

This, and I don't think it's an April Fools' 😄

Source.