问题:

I would like to get a list of only the staged filenames. I can't find the equivalent flag for --name-only for the git status command. What is a good alternative?

The file list will be piped to php -l (PHP lint syntax checker).

Solution: the complete command

git diff --name-only --cached | xargs -l php -l

回答1:

Use git diff --name-only (with --cached to get the staged files)

回答2:

The accepted answer won't let you know what kind of changes were there.

Yes, If you are not syntax checker but an ordinary person with a repository full of unstaged files, and you still want to know what will happen to staged files - there is another command:

git status --short | grep '^[RAD]'

which leads to something like:

D  dir/deleted_file
A  dir/new_file
R  dir/renamed -> dir/renamed_to

Obviously, this files were staged, and after git commit:
deleted_file will be deleted,
new_file will be added,
renamed_file will become a renamed_to.

Here is an explanation of short-format output: https://git-scm.com/docs/git-status#_short_format