Updated AMQP.php Stub

pdezwart branch added comments to the constants in AMQP.php

https://github.com/pdezwart/php-amqp/commit/9e77a45c0d23b8d291c1552c02d32723fbc5b094
see https://github.com/empi89/php-amqp-stubs/pull/3, the if should stay here.