WPLookup

Enter a search query (normally a function or variable name) and you will get a listing of multiple results:

1. Link to the page in the Function/Template Tag Reference from the WordPress Codex
2. Link to a PHPXref page for your query (@ xref.yoast.com)
3. Link to the phpDoc page at WordPress.org
4. Finally, you’ll also get a list of resources and pre-populated search links to other Codex documentation and Google web search

Editor’s note: 404 link removed.