Read the value of a class constant.

This is the same as just typing self::CONSTANTNAME, but throws a more useful message if the constant is not defined and allows the constant to be limited to a maximum length.

Should this check execute before we load configuration?

The majority of checks (particularly, those checks which examine configuration) should run in the normal setup phase, after configuration loads. However, a small set of critical checks (mostly, tests for PHP setup and extensions) need to run before we can load configuration.

Test if we've survived through setup on at least one normal request without fataling.

If we've made it through setup without hitting any fatals, we switch to render a more friendly error page when encountering issues like database connection failures. This gives users a smoother experience in the face of intermittent failures.