According to this recent post to his blog Zack Curl is a big proponent of using web application frameworks for development:

Most all PHP developers have worked off of their own code - they create all their own classes, sometimes borrow a little from previous projects, and basically create whole new frameworks without really knowing it. Even though most developers have the mindset that they have more flexibility, less redundancy in code, and other reasons that usually branch off of the two reasons, a framework actually makes your application load faster, work better, and work more effectively.

His reasons for the recommendation are:

• Classing and OOP Principles
• Model-View-Controller Schema
• Pre-built Libraries
• Built for speed

As far as his choice of a specific framework? He would opt for CodeIgniter, the lightweight and speedy product of Ellis Labs.