e configuration data.
	 *
	 * @param string|array $key   The key or array of keys to set.
	 * @param mixed        $value The value to set.
	 *
	 * @return mixed The value that was set.
	 */
	public function set( $key, $value ) {
		if ( ! is_array( $key ) ) {
			$this->_data[ $key ] = $value;
		} else {
			// set extension's key.
			$key0 = $key[0];
			$key1 = $key[1];

			if ( ! isset( $this->_data[ $key0 ] ) || ! is_array( $this->_data[ $key0 ] ) ) {
				$this->_data[ $key0 ] = array();
			}

			$this->_data[ $key0 ][ $key1 ] = $value;
		}

		return $value;
	}

	/**
	 * Checks if the current configuration is a preview.
	 *
	 * @return bool True if preview mode is enabled, false otherwise.
	 */
	public function is_preview() {
		return $this->_preview;
	}

	/**
	 * Checks if the current configuration is the master configuration.
	 *
	 * @return bool True if the configuration is the master, false otherwise.
	 */
	public function is_master() {
		return $this->_is_master;
	}

	/**
	 * Checks if the configuration is compiled.
	 *
	 * @return bool True if the configuration is compiled, false otherwise.
	 */
	public function is_compiled() {
		return $this->_compiled;
	}

	/**
	 * Sets the default configuration values.
	 *
	 * @return void
	 */
	public function set_defaults() {
		$c           = new ConfigCompiler( $this->_blog_id, $this->_preview );
		$this->_data = $c->get_data();
	}

	/**
	 * Saves the current configuration.
	 *
	 * @return void
	 */
	public function save() {
		if ( function_exists( 'do_action' ) ) {
			do_action( 'w3tc_config_save', $this );
		}

		$c = new ConfigCompiler( $this->_blog_id, $this->_preview );
		$c->apply_data( $this->_data );
		$c->save();
	}

	/**
	 * Checks if a configuration key is sealed (immutable).
	 *
	 * @param string $key The configuration key to check.
	 *
	 * @return bool True if the key is sealed, false otherwise.
	 */
	public function is_sealed( $key ) {
		if ( $this->is_master() ) {
			return false;
		}

		// better to use master config data here, but its faster and preciese enough for UI.
		return ConfigCompiler::child_key_sealed( $key, $this->_data, $this->_data );
	}

	/**
	 * Exports the current configuration as a JSON string.
	 *
	 * @return string The configuration data as a JSON string.
	 */
	public function export() {
		if ( defined( 'JSON_PRETTY_PRINT' ) ) {
			$content = wp_json_encode( $this->_data, JSON_PRETTY_PRINT );
		} else {
			$content = wp_json_encode( $this->_data );
		}

		return $content;
	}

	/**
	 * Imports a configuration from a file.
	 *
	 * @global $wp_filesystem
	 * @see get_filesystem_method()
	 *
	 * @param string $filename The path to the file to import.
	 *
	 * @return bool True if the import was successful, false otherwise.
	 */
	public function import( string $filename ): bool {
		if ( 'direct' !== \get_filesystem_method() ) {
			return false;
		}

		// Initialize WP_Filesystem.
		global $wp_filesystem;
		WP_Filesystem();

		if ( $wp_filesystem->exists( $filename ) && $wp_filesystem->is_readable( $filename ) ) {
			$content = $wp_filesystem->get_contents( $filename );
			if ( \substr( $content, 0, 14 ) === '