[BUGFIX] Use self:: instead of static:: late binding for private constants

[NamelessCoder]

Class constants are marked private but consuming code uses late static binding with static:: to access them. Since the constants are private, late static binding makes no sense (no subclass would be able to use the constants) and simply causes an error (access to undefined constant). This patch prevents that error from happening in e.g. https://github.com/georgringer/oidc-be.

There are two ways to solve this:

• Declare the constants protected or public.
• Or do not use late static binding.

I've chosen the latter in this case since there will not be a need to change the values of these constants or access them externally. Any subclass that chooses to override the method that does use the constants will have to return these values in another way or call the parent:: method to perform the initial validation and get a return code based on the constants.

[liayn]

Thanks Claus.