I have been unable to find an answer via search.

I have a website (I do not administer the servers) where the server will serve a different file than the one requested. I first noticed this when using a filename of the following form:

_foo.php (single underscore)

If I request foo.php (does not exist), the server returns _foo.php. By "returns" I mean that the server decides I meant _foo.php, processes the php file, and serves the output.

If I request afoo.php, zfoo.php, or even __foo.php (two underscores) (these files do not exist) the server returns _foo.php.

If I request aafoo.php, the server returns 404.

To sum up: the server seems to be doing a partial filename match.

My question is: what is happening and is this accepted behavior for a web server (or standard behavior of a common mod/package/etc)?