If you're just implementing a website, you could use a content management system (CMS) like Joomla, WordPress or Drupal.
Drupal is probably the most powerful and sophisticated of these 3 i.e. you can add a lot of clever components etc yourself, but of course it's also more complex. It is well documented and has been around a long time so there are plenty of people out there with Drupal skills. WordPress is the easiest of these 3 to learn, but is less sophisticated/flexible than Drupal for example. Joomla is somewhere between the other 2, in terms of flexibility/complexity.
All 3 of these have a huge number of 3rd party modules available to provide all kinds of extra functionality.
Of course, there are lots of other PHP-based CMS out there, but I've only used the above 3, which are all open source.
But if you are implementing an actual web application yourself, then I can strongly recommend Zend Framework for high quality, robust, OO PHP components e.g. it includes components for 3-tier application development, such as DAOs, Struts-like MVC, and much more. It is very flexible i.e. you can just use the components you need, and very well documented. Zend Framework is an open source project maintained by Zend, who are The PHP People as you probably know.
There's also a Zend Eclipse derivative called Zend Studio with lots of built-in support for Zend Framework, and Zend also market Zend Server i.e. a PHP/Zend Framework-based web application server. Zend Studio and Zend Server are commercial products, although I think there is a trial version of Zend Studio you can download for free.
There are other PHP-based application development frameworks out there, but I've only used Zend Framework, with Zend Studio, both of which I found to be excellent. I didn't use the Zend Server, so I can't comment on that.
But if you are implementing an actual web application yourself, then I can strongly recommend Zend Framework for high quality, robust, OO PHP components e.g. it includes components for 3-tier application development, such as DAOs, Struts-like MVC, and much more.