I think this is an Apache issue, but I'm using php pages on an Apache
server...

The problem is that when a url such as:

example.com/?page=whatever

is url encoded to:

example.com/%3Fpage%3Dwhatever

then it appears that Apache is not seeing the %3F as a ? parameter marker,
but rather as the first character of a filename (but in an encoded form).

So, if I create a file called ?.php and then use the url:
example.com/%3F.php

the actual file called ?.php is called.

What do I do to stop this?