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The idea behind a framework is to offer a design you can use across multiple applications. All applications have a number of basic things in common. A framework is designed to provide a structure for those common elements (database interaction, presentation layer, application logic) so you spend less time writing up database interface code or presentation-layer interfaces and more time writing the application itself. The architecture represented by breaking an application up in this fashion is referred to as Model-View-Controller (MVC). Model refers to your data, View to your presentation layer, and Controller refers to the application or business logic.
Recommend the top 10 projects:

1 Yii – Yii Framework: Best for Web 2.0 Development

2 CodeIgniter - CodeIgniter: an open source Web Application Framework that helps you write PHP programs.

3 Zend - A bottom-up rewrite of the PHP scripting engine. It features a modular architecture, vastly improved performance, and greater scalability.

4 CakePHP - Official website. Offers a manual for beginners and links towards the last version.
5 Symfony - Easy to use framework to build robust web applications for enterprises.

6 PHPDevShell - PHPDevShell focuses on you getting the job done, providing a highly organized environment and structure.

7 Prado - PRADO is an event-driven and component-based framework for Web application development in PHP 5.

8 Akelos - Port of Ruby on Rails development framework and designed to work for PHP4 and PHP5.

9 QPHP - QPHP is an event-driven(EDP), component-based web framework similar as architecture to ASP.NET.

10 ZooP - An Object Oriented Framework for PHP, Zoop Framework caters to the programmer and includes ajax and validation.
All the above php frameworks are listed on PHPKode.com
Best regards!
Anny!