The Developer Tutorials blog has posted a tutorial showing you how to set up a tag cloud for your site:

Every major website seems to have a tag cloud. Users love tag clouds; they help navigate masses of content quickly and easily. [...] How do we actually build a tag cloud at application level? In this tutorial, I'll take you through putting together a full-blown, calculated web 2.0 tag cloud in PHP.

They start at the end, with an example cloud with some sample tags ("php" being the largest, of course). The code for it is pretty simple - it looks at an array of counts, the number of times the tag is used, and builds the links with the correct font size automatically.

The Zend Developer Zone has a new post referencing one of the featured classes from the PHPClasses.org website - one that can create an automatic tag cloud for just about any site out there.

But how can you build a tag cloud for a site that did not have any kind of content tagging or categorization? This is a problem that can be solved by the Automatic Keyword Generator class written by Ver Pangonilo from the Philippines.

The ZDZ post includes an example of the class' usage - a pretty simple example of how it parses a string of data and, using a set of rules, parses through it to create the tags for the information.