Common scenario's:

1. The time comes for a CDN and all images need to be served up with a URL like cdn.example.com.
2. The use of SSL is very common for authentication and other sensitive user information. The problem is that SSL is much slower than a normal http request. Traffic needs to be segregated by changing the SSL URL's from https://www.example.com to https://secure.example.com.

The solution to these problems is quite trivial: simply prepend a domain to a relative URL. Consider the following config file:

[development]
site.cdn = "http://dev.example.com"
site.ssl = "https://dev.example.com"

[production]
site.cdn = "http://cdn.example.com"
site.ssl = "https://secure.example.com"

This configuration uses special URL values for a production environment, but uses the standard development server URL so the developers can still develop. A simple addition to the php bootstrap file can set up defines to use in html templates.

<?php
define('CDN', $config->site->cdn);
define('SSL', $config->site->ssl);

And then in a .phtml file you can simply do the following:

<img src="<?=CDN>/images/hero.png">
 
<a href="<?=SSL>/login.php">Login</a>

Consider prepending the domain for all URL's in your application, not just those types listed above. There are plenty of scenario's that may require AJAX calls or even normal GET/POST request to use different domains.