From a new post on PHP Magazine's site today:

In a new story on NewsForge, Cal Henderson Flickr's Web development lead talks about how Flickr is powered by a catalog of open source tools: Red Hat Linux, the Apache Web server, MySQL databases, PHP, Perl, Smarty templates, Postfix mail gateways, and even ImageMagick to handle the image manipulation. According to Henderson, PEAR and Smarty for all the templating." He estimates that they use about 20,000 lines of open source code, compared to about 60,000 lines of in-house. He goes on to talk about how PHP scripting helped Flickr find a solution to cameraphone users who wished to e-mail photos directly to their Flickr account. Read the full story.

Flickr is all about supporting the Open Source community as well - so much so that they recently opened up an API for web services. It allows users to interface quickly and easily with the photos, users, and groups that are on the Flickr website. There's even been some cool tools already generated from this API such as a GTK upload tool to scripts that create mosaics based on searching for photos on Flickr.