Safely run a function in Drupal as another user.

drupal_run_as.php

<?php
function drupal_run_as($user, $func, $arg1) {
  global $user;
  $original_user = $user;
  $old_state = drupal_save_session();
  drupal_save_session(FALSE);

  $args = func_get_args();

  $user = array_shift($args);
  $func = array_shift($args);
  $r = call_user_func_array($func, $args);
  $user = $original_user;
  drupal_save_session($old_state);
  return $r;
}