Is there a way to just commit one file type extension from a folder?

For me, just git add *.c didn’t work. I encountered fatal error like this:

git add *.php
The following paths are ignored by one of your .gitignore files:
wp-config.php
Use -f if you really want to add them.
fatal: no files added

So, in this variant git tried to add ignored files.

But such variant:

git add \*.php

worked flawlessly.
By the way, you can do git diff the same way:

git diff -- \*.php