PHPSessionHandler: Workaround PHP5 bug
PHP5 has a bug in handling boolean return values for
SessionHandlerInterface methods, it expects 0 or -1 instead of true or
false. See https://wiki.php.net/rfc/session.user.return-value.
PHP7 and HHVM are not affected.
No tests are added here because the only case where it actually makes a
difference is a can-never-happen branch.
Also, since I'm touching it already, add a @codeCoverageIgnore for the
code no longer tested thanks to I6e153ec8.