CakePHP is a rapid development framework for PHP which uses commonly known design patterns like ActiveRecord, Association Data Mapping, Front Controller and MVC. CakePHP provide a structured framework that enables PHP users at all levels to rapidly develop robust web applications, without any loss to flexibility.
CodeIgniter is an Application Development Framework - a toolkit - for people who build web sites using PHP. Its goal is to enable developer to develop projects much faster than developer could if you were writing code from scratch, by providing a rich set of libraries for commonly needed tasks, as well as a simple interface and logical structure to access these libraries. CodeIgniter lets you creatively focus on your project by minimizing the amount of code needed for a given task.
Symfony is a web application framework for PHP5 projects. It aims to speed up the creation and maintenance of web applications, and to replace the repetitive coding tasks by power, control and pleasure. Symfony is aimed at building robust applications in an enterprise context. This means that you have full control over the configuration: from the directory structure to the foreign libraries, almost everything can be customized. To match your enterprise's development guidelines, symfony is bundled with additional tools helping you to test, debug and document your project.
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.