Although I've only recently started using AJAX, I would say learn PHP (or a similar technology, I use Java Servlets) first. AJAX sends a request to the server, so you need something to process that request. You don't need to be a PHP expert to use it (with AJAX), but you do need some basic knowledge.
Most certainly learn PHP first. At the very least, you can start writing an application and not even worry about Ajax, and when you're done still have a fully working app with no Ajax whatsoever.
We really need to think about what makes sense when it comes to using these technologies. If there is a need for it in your application use it. It all comes down to what you want your application to do. Do page refreshes bother you? Are the page refreshes taking a long time? Or do you just wanna say, "yeah, my app uses some ajax", etc.
Ajax is a great tool, but don't over engineer to use it simply because its "cool." Learn the server side language first, then learn the other ways to enhance the application you've written with things like Ajax.
Just my 2 Mhz
Ryan Headley<br /><a href="http://www.sudovi.com" target="_blank" rel="nofollow">http://www.sudovi.com</a>
I would also go for PHP first as it will represent the server side backend of each application. Next, you can start with AJAX. As the AJAX applications become more complex you will need to extend your PHP and AJAX knowledge.