If you check out the source of _wp_link_page
, you’ll see all the URL calculations are coded directly within – you can either lift this code straight out into your own function, or sprinkle a little regex around the existing function:
function wpse_204737_get_post_page_url( $i ) {
if ( preg_match( '/href="https://wordpress.stackexchange.com/questions/204737/([^"]+)"https://wordpress.stackexchange.com/", _wp_link_page( $i ), $match ) )
return $match[1];
}
Worth noting that _wp_link_page
is a “private” function, and is not intended for use in 3rd party plugins & themes – it may be renamed/removed/deprecated in a subsequent release.