How can I use grep to show just filenames on Linux?

For a simple file search you could use grep's
-l and -r
options:

grep -rl
            "mystring"

All the
search is done by grep. Of course, if you need to select files on some other parameter,
find is the correct solution:

find
            . -iname "*.php" -execdir grep -l "mystring" {}
            +

The
execdir option builds each grep command per each directory, and
concatenates filenames into only one command
(+).