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<?php
/* ========================================================================= */
/* !BIDIRECTIONAL RELATIONSHIP */
/* ========================================================================= */
function bidirectional_acf_update_value( $value, $post_id, $field ) {
	// vars
	$field_name  = $field['name'];
	$global_name = 'is_updating_' . $field_name;
	if ( ! empty( $GLOBALS[ $global_name ] ) ) {
		return $value;
	}
	$GLOBALS[ $global_name ] = 1;
	if ( is_array( $value ) ) {
		foreach ( $value as $post_id2 ) {
			$value2 = get_field( $field_name, $post_id2, false );
			if ( empty( $value2 ) ) {
				$value2 = array();
			}
			if ( in_array( $post_id, $value2, true ) ) {
				continue;
			}
			$value2[] = $post_id;
			update_field( $field_name, $value2, $post_id2 );
		}
	}
	$old_value = get_field( $field_name, $post_id, false );
	if ( is_array( $old_value ) ) {
		foreach ( $old_value as $post_id2 ) {
			if ( is_array( $value ) && in_array( $post_id2, $value, true ) ) {
				continue;
			}
			$value2 = get_field( $field_name, $post_id2, false );
			if ( empty( $value2 ) ) {
				continue;
			}
			$pos = array_search( $post_id, $value2, true );
			unset( $value2[ $pos ] );
			update_field( $field_name, $value2, $post_id2 );
		}
	}
	$GLOBALS[ $global_name ] = 0;
	return $value;
}
add_filter( 'acf/update_value/name=YOUR_FIELD_NAME_HERE', 'bidirectional_acf_update_value', 10, 3 );