This thesis seeks to improve the development process of open source content management system extensions with particular focus on functional extensions of Joomla CMS. It starts by analyzing current open source CMS market to conclude that most of it is dominated by systems implemented in the PHP programming language. Standard tools used throughout the PHP development process are then analyzed to show that fully-featured integrated development environments are not used as often as with compiled languages. It's shown, however, that modern open source IDEs can deliver performance gains even when used for development in a scripting language. The focus is then shifted from general purpose PHP development to the specifics of CMS extension development. Three major open source systems (WordPress, Drupal and Joomla) are presented with focus on their extensibility. It's demonstrated that some features of Joomla's module system, which are not present in the other systems in comparison, are making it much harder to establish an efficient development workflow that would take full advantage of an IDE. Two recommended workflows for Joomla extension development are then presented while pointing out their disadvantages. Based on this analysis, a new development workflow is designed incorporating yet-to-be-developed functional extensions of Eclipse IDE to solve the problems. The Eclipse platform is then described, pointing out some key technologies used to implement these functional extensions. Finally, major components of the implementation are described.