From the PHP-Learn-It.com website, there's a quick tutorial on how to create a bit of edit in place functionality with some simple Javascript and PHP.

Suppose we have an article page on our site where people can edit articles. The article has an edit and save button which allows the user to edit the article and save the changes as necessary. View the ajax_edit_in_place demo to see what I'm talking about.

They've gone with Prototype to handle the AJAX connection and provide both the Javascript and PHP code for editing, saving and canceling an edit on the page's content.