Extending the art & spirit of PHP, Zend Framework is based on simplicity, object-oriented best practices, corporate friendly licensing, and a rigorously tested agile codebase. Zend Framework is focused on building more secure, reliable, and modern Web 2.0 applications & web services, and consuming widely available APIs from leading vendors like Google, Amazon, Yahoo!, Flickr, as well as API providers and cataloguers like StrikeIron and ProgrammableWeb.
Symfony is aimed at building robust applications in an enterprise context, and aims to give developers full control over the configuration: from the directory structure to the foreign libraries, almost everything can be customized. To match enterprise development guidelines, Symfony is bundled with additional tools to help developers test, debug and document projects. (wikipedia)
The state-of-the-art structure of the system offers a high security standard and allows you to develop search engine friendly websites that are also accessible for people with disabilities. Furthermore, the system can be expanded flexibly and inexpensively. Easy management of user rights, the Live Update Service, the modern CSS framework and many already integrated modules (news, calendar, forms, etc.) have quickly made Contao one of the most popular open source content management systems on the market.