I am running Ubuntu 14.04 x64.

I have enabled mod_rewrite on my droplet.
This works (i.e. correctly redirects to testfile.php):

RewriteRule ^testfile2$ testfile.php

But this doesn't (gives me a 404 error):

RewriteRule ^testfile$ testfile.php

So I essentially cannot redirect a URL to a file of the same name but with a PHP extension. The two examples are both working locally with MAMP.
What could be causing this issue?