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.
These are posts from previous incarnations of my blog. They catalog old writing, (abandoned) projects, and else-wise content I have written in the past that others have found engaging or useful. You'll notice the sidebars reflect the content of the archival posts rather than my current blog; for current posts click on the logo above.