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