PHP—“Defined or Exit” at the beginning of many files?

Question

I have been inspecting some PHP source codes and I more often than not find files starting with

defined(\'__someconstant__\') or exit();

I

Answer 1

Adding to @NullUserException's answer...

Yes there are other ways of preventing a file from being accessed directly (.htaccess being one), but for software that is shared with a wide audience, you can't really rely on those technologies being there. You can rely on a simple condition at the top of the files though.