R1:05eb77c0a704

Mark redirects to php.net from symbols as external

Summary: Fixes T5942. These are external but currently unmarked.

Test Plan: Visited link, got redirected.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5942

Differential Revision: https://secure.phabricator.com/D10332