We recently had a project at work that involved replacing all the relative URL's from the application with absolute URL's. In the past, developers had just hard-coded an absolute URL only when they need to force the browser over to https. Now we are using multiple subdomains, so this approach is no longer sufficient. We also wanted a way to easy rotate assets through multiple CDN URL's to speed up the time it takes a user's web browser to load all the content.
It has been said that all languages, over time, implement a dialect of lisp. PHP appears to be no exception.