use lowercase_and_underscore instead of String.lowercase

Summary: ^^^^ in graph_code_php

Test Plan:
cd tests/php/codegraph
make
should not have error about "x-str" vs "x_str"

Reviewers: pad, pieter, mathieubaudet

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D869340