On the SmartyCode.com site, there was a new article posted recently looking at making the output of your Zend Framework application XHTML compliant.

Serving XHTML is often misunderstood by php developers. Frontend engineers simply include the XHTML doctype to their documents, without actually serving document as XHTML. This triggers majority of the browsers to treat such pages as 'tag-soup'. [...] This front controller plugin's code mostly takes concepts from the excellent article by Keystone Websites, but implements in Zend Framework environment in an object-oriented way.

The code works as a plugin to the controller and runs a few checks on the contents of the data being pushed out (dispatchLoopShutdown) and returns the correct header information (DOCTYPE, language attribute) in the correct XHTML format for the data. When the plugin is registered, all it takes is a call to the "doctype()" method to output the correct information.