Add a new feed type like /atom1/.

Signature

add_feed( $feedname, $function )
feedname
 (string)
function
 (callback) Callback to run on feed display.

Return

(string) Feed action name.

Source

function add_feed($feedname, $function) {
	global $wp_rewrite;
	if ( ! in_array($feedname, $wp_rewrite->feeds) ) //override the file if it is
		$wp_rewrite->feeds[] = $feedname;
	$hook = 'do_feed_' . $feedname;
	// Remove default function hook
	remove_action($hook, $hook);
	add_action($hook, $function, 10, 1);
	return $hook;
}
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