Fix PHT linter

Summary: Variables in strings and concatenation lists.

Test Plan:
New unit tests.

  preg_match('/^((?>[^$\\\\]*)|\\\\.)*$/s', str_repeat('a', 1e6));

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4540