What serial data should I put during zone insert for remote api?

Hi,

Excuse my ignorance but what data should I input in serial field for remote API?

Code:

	//* Set the function parameters.
	$client_id = 1;
	$params = array(
			'server_id' => 1,
			'zone' => 1,
			'name' => strval($NAME),
			'type' => 'cname',
			'data' => strval($DATA),
			'aux' => '0',
			'ttl' => intval($TTL),
			'active' => strval($ACTIVE),
			'stamp' => date('Y-m-d H:i:s'),
			'serial' => '1',
			);

Thanks in advance