Not a PHP Framework but a fantastic front end framework for web development hence worth a mention here. Bootstrap was developed by Twitter as a framework to help produce consistency across their internal web design tools.
Since version 2.0 it also supports responsive design. This means the layout of web pages adjusts dynamically, taking into account the characteristics of the device used (PC, tablet, mobile phone). It's now up to version 3.0. Bootstrap is open source and available on GitHub. Developers are encouraged to participate in the project and make their own contributions to the platform. We use Bootstrap to underpin much of our CSS. For a few samples of sites using bootstrap see our client work page.