Add a PHPCS Ruleset That Can Identify Incompatibilities with Playground

[akirk]

Let's investigate whether we can identify problems with PHP code (for example: plugins) on a PHPCS basis without running through all code paths in Playground. This ruleset needs to be updated as Playground evolves, but it can serve as a tool to identify potential issues.

Additionally, we could consider implementing PHP fixers (phpcbf) for problems that can be easily avoided.

Examples of rules:

• Incompatible (My)SQL
• Unavailable PHP modules

[adamziel]

@bgrgicak let's find or start one central place, e.g. a tracking issue, to list all this compatibility-related work, prioritize, figure out where we are, where we're going etc.

Related issues that caught my eye:

• Consider maintaining stats for feature usage and kinds of errors #1736
• Quantitative logging of errors and plugins #1778

[bgrgicak]

I created a temporary milestone for compatibility tasks.

@adamziel Would you like me to start a discussion next week about this project?