I've been enums lately instead of relying on class constants. The myclabs/php-enum package provides a nice implementation. The readme lists the benefits of doing so:

Using an enum instead of class constants provides the following advantages:
  • You can type-hint: function setAction(Action $action) {
  • You can enrich the enum with methods (e.g. format, parse, …)
  • You can extend the enum to add new values (make your enum final to prevent it)
  • You can get a list of all the possible values (see below)
This Enum class is not intended to replace class constants, but only to be used when it makes sense.

Check it out: https://github.com/myclabs/php-enum