inc/functions/extvalue.php
extvalue(name, field, separator, template, header, footer) gets the values of a field in a page
Use this, if you need a field from another page and you know the name of the other page.
If you need a field from the same page, use the value function. If you don't know the name, use the query function to search for it.
The arguments separator, template, header and footer are optional. If not empty, the fields use the provided template, are separated by separator, preceeded by header and followed by footer.
User rights: If used in a normal page, the function has access to pages in transcluded namespaces. If used in a template page, the function has access to all pages.