This wiki supports InterWiki links in a style copied from UseMod: wiki. Links to pages in other wikis can be made without having to know or type the full URLs, for example: MeatBall:InterWiki will link to the page named "InterWiki" at Meatball wiki.
The InterWiki map is taken from the the <verbatim> block in InterWikiMap, but that page must be locked for it to work. This denies a potential hacker the ability to nefariously change every InterWiki link to point to some evil URL. If no map is found in InterWikiMap (or the page is not locked), PhpWiki will fall back to using the file lib/interwiki.map in your phpwiki distribution. The InterWiki map file in use at PhpWiki: can be viewed at http://phpwiki.sf.net/interwiki.map.
The map file is manually updated. It is based upon and periodically synchronized with UseMod's InterMap but it is not quite identical. One notable difference from UseMod: is that PhpWiki supports '%s' within the URLs in the map--see the entry for RFC for an example of how this works.
Not all of the entries in the InterWikiMap are actual wikis. For example, these are just regular web sites: Dictionary:fungible, JargonFile:Alderson loop, IMDB:Roman Holiday, RFC:2822, and ISBN:020171499X.
The "Category" moniker is a special entry which allows one to link a wiki page to a Category page without creating a back-reference. Thus Category:Category links to the CategoryCategory page, yet this page won't show up in the back-links listing of CategoryCategory.
The term "InterWiki" also refers to a broader concept. See PhpWiki:InterWikiSearch for more links.