On the tech.pro site there's a recent tutorial posted showing you a basic way to create an RSS feed using data coming from a database accessed via PDO.

Using an RSS feed on your website is a great way of letting your visitors, search engines or directories get a hand on your content. RSS feeds are common practice on most blog and CMS platforms including Wordpress, Joomla and evenly the newly released Ghost. If you're using a CMS or similar platform, the likelihood is that you don't need to implement an RSS feed yourself. [...] Below you've got the step-by-step process to create anything from the simple, standard-compliant RSS feed - up to the more advanced.

The tutorial shows you how to pull the data from a simple database table (SQL not provided, but pretty easy to figure out(, including example PDO connections for several database types. This data is then manually appended into an XML string to build out the RSS feed correctly. They also talk about implementing the Dublin Core metadata as a way for providing more information about the feed and its contents (including an image and category details).