Yii is a BSD-style-licensed web-development framework for PHP. Reading from the homepage
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Yii is a high-performance component-based PHP framework for developing large-scale Web applications. Yii is written in strict OOP and comes with thorough class reference and comprehensive tutorials. From MVC, DAO/ActiveRecord, widgets, caching, hierarchical RBAC, Web services, to theming, I18N and L10N, Yii provides nearly every feature needed by today's Web 2.0 application development. And all these come without incurring much overhead. As a matter of fact, Yii is one of the most efficient PHP frameworks around.