User:GeorgeMoney/Help/ExternalEditor

The reason it's trying to download an "index.php" is because it is set to do it in your preferences (maybe you were experimenting and accidentally set it?), and there is an easy solution.

Just go to Special:Preferences.

Once you are there you should click on the "editing" tab:

.

Next, scroll down to the bottom and uncheck the box that says "use external editor by default": .

Once you press save, you should be able to edit pages normally again.

I hope this helps!