import attachment to gallery

add_action(
    'wp_ajax_mpp_custom_move_media_to_gallery',
    function () {

        // this is a sample, use your own nonce verification.
        // Step 1. Please uncomment and validate nonce
        // if ( ! wp_verify_nonce( $_POST['_wpnonce'], 'mpp-custom-move-media-to-gallery' ) ) {
        // wp_send_json_error( __( 'Invalid Nonce', 'mpp-custom-move-media-to-gallery' ) );
        // }

        // Step 2:- check presence of attachment & gallery
        $attachment_id = isset( $_POST['attachment_id'] ) ? absint( $_POST['attachment_id'] ) : 0;
        $gallery_id    = isset( $_POST['gallery_id'] ) ? absint( $_POST['gallery_id'] ) : 0;
        if ( ! $gallery_id || ! $attachment_id ) {
            wp_send_json_error( __( 'Invalid Request', 'mpp-custom-move-media-to-gallery' ) );
        }
        // Step 3. Check if the current user is allowed to move the attachment to gallery
        // I will live it upto you to decide the criteria.
        // I will probably check for media owner and gallery owner.

        // Step 4. If all is good, let us try to attach the media to gallery
        $result = mpp_import_attachment( $attachment_id, $gallery_id, $override = array() );

        if ( is_wp_error( $result ) ) {
            wp_send_json_error( $result->get_error_message() );
        }
        // if we are here, the ersult is our media id.
        wp_send_json_success( array( 'media_id' => $result ) );
    }
);