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    $headers = file_download_headers($uri);

/**

 * Retrieves headers for a private file download.

 *

 * Calls all module implementations of hook_file_download() to retrieve headers

 * for files by the module that originally provided the file. The presence of

 * returned headers indicates the current user has access to the file.

 *

 * @param $uri

 *   The URI for the file whose headers should be retrieved.

 *

 * @return

 *   If access is allowed, headers for the file, suitable for passing to

 *   file_transfer(). If access is not allowed, an empty array will be returned.

 *

 * @see file_transfer()

 * @see file_download_access()

 * @see hook_file_downlaod()

 */

function file_download_headers($uri) {

  // Let other modules provide headers and control access to the file.

  // module_invoke_all() uses array_merge_recursive() which merges header

  // values into a new array. To avoid that and allow modules to override

  // headers instead, use array_merge() to merge the returned arrays.

  $headers = array();

  foreach (module_implements('file_download') as $module) {

    $function = $module . '_file_download';

    $result = $function($uri);

    if ($result == -1) {

      // Throw away the headers received so far.

      $headers = array();

      break;

    }

    if (isset($result) && is_array($result)) {

      $headers = array_merge($headers, $result);

    }

  }

  return $headers;

}

/**

 * Checks that the current user has access to a particular file.

 *

 * The return value of this function hinges on the return value from

 * file_download_headers(), which is the function responsible for collecting

 * access information through hook_file_download().

 *

 * If immediately transferring the file to the browser and the headers will

 * need to be retrieved, the return value of file_download_headers() should be

 * used to determine access directly, so that access checks will not be run

 * twice.

 *

 * @param $uri

 *   The URI for the file whose access should be retrieved.

 *

 * @return

 *   Boolean TRUE if access is allowed. FALSE if access is not allowed.

 *

 * @see file_download_headers()

 * @see hook_file_download()

 */

function file_download_access($uri) {

  return count(file_download_headers($uri)) > 0;

}