Ensure displaying PHP Manual function page for PHP function symbols

Summary:
Some PHP functions have the same name as extension or class.
php.net prefers them over functions.

Test Plan:
http://www.php.net/hash
http://www.php.net/function.hash
http://www.php.net/function.hash_final

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1977