From the HashBangCode.com site today there's a new post that revisits filtering with the filter_var function included with the language. The focus in this article is specifically in validating URLs with the FILTER_VALIDATE_URL flag.
Quite a while ago I looked at using the filter_var() function to validate URL's using the FILTER_VALIDATE_URL flag and someone pointed out recently that this function has not only changed since the initial release, but that a number of flags can be added to change the way that this function works.
He lists some of the other flags that are now available that can be used in conjunction with FILTER_VALIDATE_URL to get more fine-grained in your filtering - checks on things like a required scheme, hostname and query string. He includes some code with a set of URLs to run through some tests and output as a table with the pass/fail rank of each URL value. You can see the resulting output here.