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.