Struggling through the development process the first time helped us weed through the resources available at Intuit's developer network to find the most helpful. The following links should point you to more information than you'll know how to use...

• QB Resources Thread - a compilation of resources for download in our forums.
• qbXML Onscreen Reference - A must use for developing with the Web Connector. Shows the full schema for the available qbXML messages and the possible responses.
• Web Connector Programming Guide - The official documentation on programming for the Web Connector.
• Web Connector Updates - The location to go to get updates and downloads of the Web Connector.
• Alpha Geek article on the Web Connector - The section on the WC's communication model is very helpful.
• Support Forums - Quick Books Web Connector support forum at the Intuit Developer Network.
• Intuit Developer Network home - The place to go to register for the IDN and gain access to the SDK, forums, and more!

There are programming examples and other useful resources available, but these were the most helpful for us. Please be aware that there are no PHP examples, so you should be savvy enough to know how to translate the different languages into PHP. The code samples page does have some sample qbXML requests that you might look at as you develop your application:

http://developer.intuit.com/qbsdk-current/samples/readme.html