Sorry if that last post wasn't the clearest.
If you're wanting to change the content in the div, without reloading the entire page (e.g. a frame-like effect), then it's my understanding that it cannot be done using PHP alone - no matter the source. JS and Layers would be your friends here I assume, although it's not something i've done recently.
The "Else it's straightforward" was meaning if you were happy to accept the page-reloading each time the user clicked a link, as then it'd just be a standard php cms type affair.