While not directly related to PHP, there's a new article over on DevArticles today about creating Javascript/CSS pop-up notes for your site (much the way that they use them for keywords in their articles).

From Creating Pop-Up Notes with CSS and JavaScript Part I:

A pop-up note contains information and can be made to appear when a visitor to your website moves the mouse over the appropriate area. Alejandro Gervasio explains how to create several different kinds of pop-up notes using CSS and JavaScript.

Within the structure of Web pages, pop-up notes are useful accessibility elements. They are often used to create footnotes, sidebars for articles, or to define an acronym, among other things. They're widely employed by numerous professional websites, fairly expanding their capabilities, in order to make visitors feel a little bit more comfortable during the surfing experience.

These handy little notes can be used for everything from an explaination of a simple link to getting specific on a map of the world. Unfortunately, there are a few browser-specific issues they you'll have to deal with, but overall, this little hack is a pretty cool idea...