Cannot create a final abstract class
Trying to create a final abstract class is a syntax error:
$ php -r 'final abstract class A{}'
Parse error: syntax error, unexpected T_ABSTRACT, expecting T_CLASS in Command line code on line 1
$ php -r 'abstract final class A{}'
Parse error: syntax error, unexpected T_FINAL, expecting T_CLASS in Command line code on line 1
This is useful as a container for only static methods and properties, the "OO way" of doing globals in a namespace-esque way. A more verbose and semantically-different alternative is to create a final class with a private constructor, but there's no reason to arbitrarily disallow this combination.