Manual:Hooks/InterwikiLoadPrefix

Details
Return true without providing an interwiki to continue interwiki search.
 * $prefix: interwiki prefix we are looking for.
 * &$iwData: output array describing the interwiki with keys iw_url, iw_local, iw_trans and optionally iw_api and iw_wikiid.

This hook is not called when $wgInterwikiCache is used.