Not so much a code widget, but an important part of using PHP. Using an include allows you use a given bit of content or script on multiple pages while maintaining a single file (much like CSS, essentially).
This is a chunk of standard HTML, saved as a .php file, which by itself is hardly useful. However, it is being inserted into this larger page as an include.
A similiar function is require(), which works like include but will fail if there are errors (if the file to be included doesn't exist, for example, the rest of the page won't be processed). Include will spit out the errors, but will allow the rest of the page to render.