How to ignore files which are in repository?

If the file is still displayed in the status, even though it is in the .gitignore, make sure it isn't already tracked.

git rm --cached config.php

If you just want to ignore it locally, you could also make it ignored by the git status:

git update-index --assume-unchanged config.php

As commented, do note that using --assume-unchanged might cause unwanted data loss as git stash resets the "ignored" files to the state in upstream, undoing local changes, without a warning or saving.

Ignore checked in file:

git update-index --assume-unchanged file

To revert

git update-index --no-assume-unchanged file

Revert All

git update-index --really-refresh 

If the file is already in the repository, and hence the Index/Staging area, then an update to .gitignore won't change that situation. It would keep being committed.

To remove the file from the Index/Staging area use git rm <file>.