Similar to the <abbr> </abbr> is the acronym tag: <acronym> </acronym>.
The acronym tag should be used at the very least on any site a beginner or student to the site's content would visit. It's also a good idea to use to prevent any confusion on an acronym. Most browsers will have a small, yellow tag appear (showing the contents of the title attribute) when the mouse cursor hovers over the text enclosed in the acronym tag.
For example: Am I talking about HAN or HAN? PHP or PHP?