The CodeIgniter (codeigniter.com) framework is by far the best thing I have ever used, ever. It speeds up development time in PHP so much that you can write a fully functioning blog with comments, rss and trackbacks within a day (In fact I plan to do just that tomorrow for my website).

CodeIgniter uses the MCV design pattern. This has 3 separate sections; models, controllers and views. Models are optional and deal with database connections and functions. Controllers are where all processing takes place and where most of your PHP code will end up. Views are what your application is going to output, ie. html with some PHP echoes as well. This approach makes it vastly easier to make complex applications.

CodeIgniter is highly extensible and provisions for custom libraries/plugins. You can also override or extend core classes which is particuarly useful if, for example, you find yourself making lots of MySQL join commands because you can add your own function for it to the core database class.

Codeigniter's use of URLs is really the thing I love most about it. It basically replaces the GET method of passing on variables to a more SEO way. Instead of having "example.com/blog/view.php?id=3" you can have "example.com/blog/view/3/<Post Title>".

I have been using CodeIgniter for a long time now and it continues to suprise me with what it can do. For instance I recently found out it has a built in template parser class. So go check it out and be amazed at what you can do in under an hour.