src/SelectUploadField.php
protected function folderPathFromID($folderID)
    {
        if (empty($folderID)) {
            return false;
        }
        $folder = Folder::get()->byID($folderID);
        if (!$folder) {
            return false;
        }

        // Translate path
        $path = $folder->getFilename();
        if (stripos($path, ASSETS_DIR) === 0) {
            $path = substr($path, strlen(ASSETS_DIR) + 1);
        }
        return $path;
    }
protected function folderPathFromID($folderID)
    {
        if (empty($folderID)) {
            return false;
        }
        $folder = Folder::get()->byID($folderID);
        if (!$folder) {
            return false;
        }

        // Translate path
        $path = $folder->getFilename();
        if (stripos($path, ASSETS_DIR) === 0) {
            $path = substr($path, strlen(ASSETS_DIR) + 1);
        }
        return $path;
    }
Get path of a folder relative to /assets/ by id.
This will be a format appropriate for setting setFolderName to

@param int $folderID
@return string|bool Relative path to the assets directory, or false if not found