Add note fall through is intentional.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
New directive %Core.AllowHostnameUnderscore

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Explicitly specify decorator name.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Tighten up invariants.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
PSR-2 reformatting
PHPDoc corrections