WP_Upgrader::maintenance_mode( bool $enable = false )

	public function maintenance_mode( $enable = false ) {
		global $wp_filesystem;
		$file = $wp_filesystem->abspath() . '.maintenance';
		if ( $enable ) {
			$this->skin->feedback('maintenance_start');
			// Create maintenance file to signal that we are upgrading
			$maintenance_string = '<?php $upgrading = ' . time() . '; ?>';
			$wp_filesystem->delete($file);
			$wp_filesystem->put_contents($file, $maintenance_string, FS_CHMOD_FILE);
		} elseif ( ! $enable && $wp_filesystem->exists( $file ) ) {
			$this->skin->feedback('maintenance_end');
			$wp_filesystem->delete($file);
		}
	}