bp_document_download_file( $attachment_id,  $type = 'document' )

function bp_document_download_file( $attachment_id, $type = 'document' ) {

	// Add action to prevent issues in IE.
	add_action( 'nocache_headers', 'bp_document_ie_nocache_headers_fix' );

	if ( 'document' === $type ) {

		$the_file = wp_get_attachment_url( $attachment_id );

		if ( ! $the_file ) {
			return;
		}

		// clean the file url.
		$file_url = stripslashes( trim( $the_file ) );

		// get filename.
		$file_name = basename( $the_file );

		// get file extension.
		$file_extension = pathinfo( $file_name );

		// security check.
		$file_name_lower = strtolower( $file_url );

		// Get all allowed document extensions.
		$all_extensions                   = bp_document_extensions_list();
		$allowed_for_download             = array();
		$allowed_file_type_with_mime_type = array();
		foreach ( $all_extensions as $extension ) {
			if ( isset( $extension['is_active'] ) && true === (bool) $extension['is_active'] ) {
				$extension_name                                      = ltrim( $extension['extension'], '.' );
				$allowed_for_download[]                              = $extension_name;
				$allowed_file_type_with_mime_type[ $extension_name ] = $extension['mime_type'];
			}
		}

		$whitelist = apply_filters( 'bp_document_download_file_allowed_file_types', $allowed_for_download );

		$file_arr = explode( '.', $file_name_lower );
		$needle   = end( $file_arr );
		if ( ! in_array( $needle, $whitelist ) ) {
			exit( 'Invalid file!' );
		}

		$file_new_name = $file_name;
		$content_type  = isset( $allowed_file_type_with_mime_type[ $file_extension['extension'] ] ) ? $allowed_file_type_with_mime_type[ $file_extension['extension'] ] : '';
		$content_type  = apply_filters( 'bp_document_download_file_content_type', $content_type, $file_extension['extension'] );

		bp_document_download_file_force( $the_file, $file_name );
	} else {

		// Get folder object.
		$folder = folders_get_folder( $attachment_id );

		// Get Upload directory.
		$upload     = wp_upload_dir();
		$upload_dir = $upload['basedir'];

		// Create temp folder.
		$upload_dir = $upload_dir . '/' . $folder->user_id . '-download-folder-' . time();

		// If folder not exists then create.
		if ( ! is_dir( $upload_dir ) ) {

			// Create temp folder.
			wp_mkdir_p( $upload_dir );
			chmod( $upload_dir, 0777 );

			// Create given main parent folder.
			$parent_folder = $upload_dir . '/' . $folder->title;
			wp_mkdir_p( $parent_folder );

			// Fetch all the attachments of the parent folder.
			$get_parent_attachments = bp_document_get_folder_attachment_ids( $folder->id );
			if ( ! empty( $get_parent_attachments ) ) {
				foreach ( $get_parent_attachments as $attachment ) {
					$the_file  = get_attached_file( $attachment->attachment_id );
					$file_name = basename( $the_file );
					copy( $the_file, $parent_folder . '/' . $file_name );
				}
			}
			bp_document_get_child_folders( $attachment_id, $parent_folder );

			$zip_name  = $upload_dir . '/' . $folder->title . '.zip';
			$file_name  = sanitize_file_name( $folder->title ) . '.zip';
			$rootPath  = realpath( "$upload_dir" );

			$zip = new ZipArchive();
			$zip->open( $zip_name, ZipArchive::CREATE | ZipArchive::OVERWRITE );

			$files = new RecursiveIteratorIterator( new RecursiveDirectoryIterator( $rootPath ), RecursiveIteratorIterator::LEAVES_ONLY );

			foreach ( $files as $name => $file ) {
				$filePath     = $file->getRealPath();
				$relativePath = substr( $filePath, strlen( $rootPath ) + 1 );

				if ( ! $file->isDir() ) {
					$zip->addFile( $filePath, $relativePath );
				} else {
					if ( $relativePath !== false ) {
						$zip->addEmptyDir( $relativePath );
					}
				}
			}

			$zip->close();

			header( 'Expires: 0' );
			header( 'Cache-Control: no-cache, no-store, must-revalidate' );
			header( 'Cache-Control: pre-check=0, post-check=0, max-age=0', false );
			header( 'Pragma: no-cache' );
			header( 'Content-type: application/zip' );
			header( "Content-Disposition:attachment; filename={$file_name}" );
			header( 'Content-Type: application/force-download' );

			readfile( "{$zip_name}" );

			BP_Document::bp_document_remove_temp_directory( $upload_dir );
			exit();

		}
	}

}