Magic links are a feature of MediaWiki core that create automatic links for three hardcoded external identifiers. As of MediaWiki 1.28, this feature is disabled by default but may be enabled using the $wgEnableMagicLinks configuration parameter.
Examples:
ISBN 0-7475-3269-9 → ISBN 0-7475-3269-9PMID 1234 → PMID 1234RFC 1234 → RFC 1234ISBN is hardcoded to link to Special:Booksources (although the content of that special page can be changed by editing MediaWiki:Booksources and MediaWiki:Booksources-text), while the URLs for PMID and RFC can be changed by editing the system messages pubmedurl and rfcurl, respectively.
Because magic links are hardcoded and inflexible, you can also use more straightforward interwiki or links:
[[Special:Booksources/0-7475-3269-9|ISBN 0-7475-3269-9]] → ISBN 0-7475-3269-9[[pmid:1234|PMID 1234]] → PMID 1234[[rfc:1234|RFC 1234]] → RFC 1234Creating local templates similar to Wikipedia's w:Template:ISBN, w:Template:IETF RFC, and w:Template:PMID or Wikiversity's v:Template:ISBN, v:Template:RFC, and v:Template:PMID may be useful.