Gone are the days of coding entire PHP apps from scratch. These are the 5 forerunners for the next generation of PHP frameworks. Each one of these frameworks has some foreword thinking quality that sets them apart from the PHP frameworks of yesterday. Many of these are a response to the recent Ruby on Rails, rapid application development hype, and some, like PHP on Trax is a direct port of Ruby on Rails. Of the frameworks listed below, I have learned AGAVI as well as Symfony. I can honestly say I don’t have the desire to build a PHP app from scratch ever again. These frameworks make it so easy to get started and have a working app you’ll want to create apps just for the hell of it.
I had the pleasure of learning and embracing the symfony framework while working at NAT1ON on an AJAX shoutbox service. Symfony boasts easy AJAX implementation and includes the entire Script.aculo.us suite of Javascript effects. Symfony also has the ability to generate propel CRUD and application scaffolding from an already constructed SQL database. That means it objectifies all the SQL language and makes creating database driven apps easy as pie.
Symfony Homepage - AJAX Screencast

CakePHP recently announced their 1.0 stable version of their framework. Cake’s strongly Object Oriented nature makes it easy for anyone who has had OO experience to pick up. Like all of the other frameworks mentioned here Cake is keen on rapid application development as well as AJAX implementation. If you have never learned a PHP framework CakePHP might be the way to go, since the influx of users surrounding its 1.0 release will ensure an active community for the next couple of months.
CakePHP Homepage-15 Minute Blog Tutorial

PHPArch.com recently nominated Zend for the best Application Dev tool and with 95,000 download Zend’s success cannot be disputed. They are also partnered with Ning.com, an online platform for painlessly creating web apps. Zend promises to be the backbone of the next generation of web applications.
Zend Homepage

Avagi is the other framework that I have personal experience with, although its a lot less fun to work with than Symfony It strongly adheres to the MVC philosophy and is a good bet for anyone who requires that type of structure. Being a branch from the popular MOJAVI project there is a relatively strong and active user base both in the forums as well as IRC channels.
Agavi Homepage - QuickStart Movie

PHP on Trax is essentially a direct port of Ruby on Rails, in fact it used to be called PHP on Rails until someone decided that “trax” and “rails” are the same thing. Like Ruby on Rails and the other frameworks here PHP on Trax adheres to the MVC principle tightly. If you are a Rails programmer and you really want to do rails like development but are stuck in a PHP environment then PHP on Trax would be your best bet.PHP on Trax Homepage

While each one of these frameworks fits a specific need they all maintain qualities that are redefining the web, making it a more usable fun place to work. Given that all these are heavily MVC, object oriented constructs they should all be easy to learn, but the truth is each one has picky nuances that don’t specifically translate from one to the other. Case in point choose one, stick to it, master it, create the next killer app.