Re: Zend framework
@ga_user: You can get any framework to do pretty much anything. Depends on how much time and money you want to spend on it. For example, you make a Google out of Zend framework but would it be the best solution? I doubt it. It will be effective but not efficient :)
@Ansoon: First of all, why do you need a framework? what are your requirements?
Frameworks are generally used by programmers to speed up development and test out certain hypothsis. Whether that solution will make it to production or not, that's another story.
PHP itself provides methods which you can use to develop websites quickly and efficiently. Popular websites like Digg, Flickr don't use any commercial/public frameworks. A framework is a joke because you can build a framework yourself while you are developing a website, it all depends on arranging files and adapting a programming style. There are tons of framework out there but they all cater to some ideology. Before riding the framework ride, have a firm grip on PHP itself. Remember, the best of programmers are those who can program without a framework. A solution you developed using the PHP would be much more faster then any framework out there. Not only that but it will scale better.
Now the right question to ask is, why do web frameworks exist?
With the hype of CMS {Content Management Systems} in 1990s, there was an outcry of building customized CMS'es for Enterprises. IBM and Microsoft were among the first enterprises to show off their feature complete in-house developed CMS'es. Soon after, everyone started writing their own CMS from scratch. To capture the markets companies started rolling out frameworks which can do the most common of the tasks in CMS'es like login.logout, session management, cookie management, etc. Hence, web frameworks were introduced.
Zend
Zend is a feature complete framework with great documentation. No. 1 customer for Zend framework are enterprises, so their development roadmap revolves around enterprise customers than an average person like you or me. This isn't a bad thing since you'll find its much more easier to integrate with other enterprise level products like Active Directory with your Application or CMS.
cakePHP
Three words used to describe cakePHP would be, hype, slow, bulky. Perhaps someone out there knows something I don't but as far as I'm concerned, cakePHP is nothing more than a hype. Since it copied Ruby on Rails and got on the MVC train, so it was hailed by the PHP developers. Whereas the truth is development in cakePHP is painful and the outcome doesn't wow anyone. Websites developed in cackPHP are slow to say the least; you have to throw in extra hardware to balance everything. Documentation is terrible and after all these years of development, no one has been able to come out with a single cakePHP book. That in itself should show you hype around it. Abandoned companies like Mambo have jumped on board to capture hype. Still kudos to the developers who made the effort of making cakePHP.
Codeigniter
A firm grip on PHP and some help from Dr. Codeigniter is all you need to develop applications or websites in a matter of hours. It’s lean, secure, requires lesser code to write a feature and above all faster then cakePHP. Great documentation, even have a few books published. Use it with Git and you'll never be unhappy developer.
Other Options
There are tons of content management system out there that provides open frameworks. Few to mention; Silverstripe, Drupal, Joomla, Plone, modX etc.
Bottomline is that everything depends on your design, and requirements. If you are developing a portal, you are much better off writing it in PHP with no framework. If you are writing an enterprise level application with not many users, Zend is your answer. If you want to write a public website and speed up development process, use Codeigniter. And if its just a some small company .com, use a CMS :)