Summary:
@chad is hitting an issue described in P961, which I think is this bug in PHP: https://bugs.php.net/bug.php?id=43200
Work around it by defining a "PHIDInterface" and having both "Flaggable" and "Policy" extend it, so that there is only one `getPHID()` declaration.
Test Plan: shrug~
Reviewers: chad, btrahan
Reviewed By: chad
CC: chad, aran
Differential Revision: https://secure.phabricator.com/D7408
