As both the Zend Developer Zone and Ajaxian point out, there's been two new projects based on the Ext JS user interface widget system adapted for PHP - PHP-Ext and ExtPHP.

PHP-Ext is an open source widget library written for PHP 4 and 5 to empower the UI Layer. It is based on Ext JS javascript widgets which provide a standard and powerful API to build Rich Internet Applications. It basically works as a convenient wrapper for the Ext JS Javascript Objects.

And from the ExtPHP site:

I needed to be the one person who would have to wrestle JavaScript when problems happened. I therefore needed a way to insulate them from the gory details, which is why I created ExtPHP. As you may have guessed from subtle hints, such as this entry's 12-feet tall header, ExtPHP creates a PHP wrapper for ExtJS.

The Ajaxian post also includes code examples for each of them, one showing form construction and the other a tabbed interface.