The [new constants][new-constants] should probably just be integers. They're values don't really have much significance, since they're meant to be compared as constants, so following the PHP (Java inspired) convention of simply [assigning an integer value][const-int-val] would be better suited here.

 [new-constants]: https://github.com/chriso/klein.php/pull/129/files#L0R33
 [const-int-val]: https://github.com/chriso/klein.php/blob/dd0831a572527929a7071792801579c27d0ff7e7/src/Klein/Klein.php#L59