Using the Factory method pattern to have different file storage mechanisms.

FileStorageFactory.php

<?php

/**
 * Class FileStorageFactory
 * A factory class that instantiates the appropriate FileStorage engine
 */
class MyProject_Service_FileStorageFactory
{
    /**
     * Available file storage mechanisms
     * I haven't implemented them all, but this gives an idea of how it 
     * can be easy to add a new completely different way of storing file
     * @var array
     */
    protected static $mappings = array(
        'local'   => 'MyProject_Service_FileStorage_Local',
        'webdav'  => 'MyProject_Service_FileStorage_Webdav',
        'sftp'    => 'MyProject_Service_FileStorage_Sftp',
        'gdrive'  => 'MyProject_Service_FileStorage_GoogleDrive',
        'dropbox' => 'MyProject_Service_FileStorage_Dropbox'
    );

    /**
     * Cached file storage object
     * @var MyProject_Service_FileStorage_Abstract
     */ 
    protected static $cache = NULL;

    /**
     * Factory method
     * 
     * @return MyProject_Service_FileStorage_Abstract
     */
    public static function factory()
    {
        if (self::$cache === NULL)
        {
            // Read the file storage type from the settings
            $type = MyProject_Configuration::get('filestorage');
            
            // Create the appropriate file storage
            if (array_key_exists($type, self::$mappings))
            {
                $className = self::$mappings[$type];
                self::$cache = new $className();
            }
            else
            {
                throw new Exception('FileStorageFactory: type ' . $type . ' not found');
            }
        }
        
        return self::$cache;
    }
}

/**
 * Abstract class FileStorage
 * A static file storage that uses the correct file storage mechanism, based on configuration
 * 
 * Sub-classes implement the methods, e.g.
 * MyProject_Service_FileStorage_Local
 * MyProject_Service_FileStorage_WebDav
 * MyProject_Service_FileStorage_Sftp
 * etc.
 */
abstract class MyProject_Service_FileStorage_Abstract
{
  /**
   * Save a file
   * @param string $filename  file name on the local file system
   * @return bool|int         ID of the filename saved, or FALSE if it failed
   */ 
  abstract public function put($filename)
  {
    //
  }
  
  /**
   * Get a file
   * @param int $id           ID of the filename
   * @return bool|array       Filename info, or FALSE if it failed
   */ 
  abstract public function get($id)
  {
    //
  }

}