Booking id as Reference Id

/*
Create placeholder for reference id which uses booking id
however this is padded by 0 and max of 10 digit.
*/
add_filter('em_booking_output_placeholder','my_em_placeholders_custom_bookingid',1,3);
function my_em_placeholders_custom_bookingid($replace, $EM_Booking, $result){
	switch( $result ){
		case '#_REFERENCEID':
			$replace = str_pad($EM_Booking->booking_id, 10, '0', STR_PAD_LEFT);
			break;
	}
	return $replace;
}