Assigning permalinks to a page in PHP (Without using htaccess redirect)?

[Ikabod]

This is a noob question I belieive, in a content management system as well as several other types of sites that work on submissions, once you submit a URL in a URL shortening website for instance, how do you use PHP to redirect to the appropriate URL without a 404 or without using an htaccess.Based on what I've found in simple url shortening scripts online, an htaccess is always used to redirect 404s to a PHP file which process the URL and goto the specific page, how do you do this without an htaccess?Another example would be any blog software, once you submit a post, if you goto the specific URL it retrieves the appropriate post without the use of an htaccess.I hope I'm being clear, thanks.