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.
Before Bootstrap, various libraries were used for interface development, which led to inconsistencies and a high maintenance burden. Bootstrap is an excellent set of carefully crafted user interface elements, layouts, and JavaScript tools, freely available to use in your next web design project.
Bootstrap was released to the world in August 2011, with the announcement explaining that its value would be to empower front-end developers to kick start projects more efficiently and effectively. The Bootstrap style sheet provides an easy-to-implement 960 grid for efficient layout, as well as expertly crafted styles for typography, navigation, tables, forms, buttons, and more. To take care of everyday JavaScript touches, Bootstrap provides a well built set of jQuery plugins for drop-down menus, tabs, modal boxes, tooltips, alert messages, and more.
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.