goodness knows what I did differently yesterday several times...
You can easily find out (if you are sufficiently interested) by typing history
in a terminal.
It'll give you the last 500 or so commands you issued.
I expect you'll notice a spelling mistake in an earlier line that refers to /usr/share/mydms/index.php
An editor that is asked to open a file that does not exist (perhaps by virtue of a mis-type) will open an empty file, assuming that that is what you wanted (quite sensible default behaviour).