Phabstractic: Restrictrictions Data Type (Predicate)
Filters are important and very useful objects, allowing values or other logic to be filtered in and out of a particular set of data. Restrictions.php offers a way to use type checking in PHP, but in a very flexible and dynamic way. This can be useful for defining lists or sets that only accept certain values, which is useful for code cohesiveness and can reduce error checking.
You can see this blog post for more information.