The Symfony Components are standalone and reusable PHP classes. With no pre-requisite, except for PHP, you can install them today, and start using them right away. Currently, there are three components available at the moment.

YAML – A PHP library that speaks YAML
Symfony YAML is a PHP library that parses YAML strings and converts them to PHP arrays. It can also converts PHP arrays to YAML strings.

Event Dispatcher – Facilitating communication between PHP classes
Symfony Event Dispatcher is a PHP library that provides a lightweight implementation of the Observer design pattern.

Dependency Injection – Reinventing how you manage PHP classes
Symfony Dependency Injection is a PHP library that provides a robust Dependency Injection Container.

Requirements: -
Demo: http://components.symfony-project.org/
License: MIT License