CodeIgniter is the only PHP framework that boasts a very small footprint, designed for smaller PHP sites, possibly using shared-hosting providers and require the least amount of overhead to create full-featured PHP web applications.
Yii is a robust, modular PHP framework that is well-suited for developing large-scale Web applications. Yii comes standard with a full host of features, including MVC, ActiveRecord, caching, AJAX libraries, authentication and authorization components, input validation, Web services, and many more.
CakePHP is a rapid application development PHP framework that provides an extensible architecture for developing, maintaining, and deploying PHP based web applications.
Zend emphasizes flexible architecture and contains a host of addons, such as Zend Server, Zend Studio, and Zend Platform
Symfony advertises itself as a framework for enterprise applications. The framework seeks to provide a high degree of control over the configuration: from directory structure to external libraries, almost all parameters can be customized. Symfony is also bundled with a host of tools for testing, debugging and documentting a web application.