Symfony, sponsored by Sensio Labs, is a well-known PHP framework that integrates a number of open source PHP projects. (For example, Symfony uses the Zend framework's logging system.) Conversely, some Symfony components are available as stand-alone libraries from the Symfony components website. The current version (1.4.8, at the time of this writing) is written in PHP 5, and requires PHP 5.2.4 or later.
Symfony demands a bit more of a learning curve than the other Web frameworks. If you're new to Symfony, working through the Getting Started tutorial is highly recommended. Fortunately, the Symfony website provides a prebuilt, easy-to-understand application called the "Symfony sandbox." Just download it, unzip it in your Web root, and it's ready to run. The application is preconfigured to use the SQLite database engine (though you can configure the application for a different database engine) and provides a fine way to explore a Symfony application without having to construct one yourself.