ACF Updates

<?php update_post_meta( $post_id, $meta_key, $meta_value, $prev_value ); ?>

//add attendee to event ateendee list (relationship)
$event_id = $_POST['post_id'];
$attendee_list = get_field( 'attendees_list', $event_id );
$new_attendee = get_post( $attendee_id );
				
if( !is_array($attendee_list) ):
	$attendee_list = array();
endif;

array_push( $attendee_list, $new_attendee );
update_field( 'field_54811a48219fd', $attendee_list, $event_id );