Yii is a free and open source framework for PHP, meaning it's a library of functions that help you get your website up and running faster.
It uses the model-view-controller pattern, which means less spaghetti code.
It has a pretty good code generator called Gii, that lets you save time writing code to connect to your database.
It uses the ActiveRecord pattern for object-relational mapping, which means less SQL to write.
It might be faster than other frameworks, as it loads components only when needed.
I also use these technologies with Yii: