Example of Drupal Batch API

drupal_batch_example.php

<?php
/**
 * Import submit function
 */
function ret_user_import_upload_form_submit($form, &$form_state) {
  if ($file = file_save_upload($source = 'ret_user_import_csv_file', $validators = array(), $dest  = FALSE, $replace = FILE_EXISTS_REPLACE)) {
    $offset = 0;
    $headers = _ret_user_import_get_row($file->filepath, $offset);
    if ($headers === FALSE) {
      // there needs to be consistency, so enforce the existence of a 'header' row
      return;
    }
    $srcinfo = new stdClass;
    $srcinfo->filepath = $file->filepath;
    $srcinfo->offset = $offset;
    $batch = array(
      'title' => t('Importing user records from %filename',
        array('%filename' => $file->filename)),
      'init_message' => t('User import initializing'),
      'error_message' => t('Import failed'),
      'progress_message' => '',
      'operations' => array(
        array('_ret_user_import_import_csv', array($srcinfo)),
      ),
      'finished' => '_ret_user_import_import_finish',
    );
    batch_set($batch);
    
    $redirect_path = 'admin/ret/import/user/overview';
    batch_process($redirect_path);
  }
  else {
    drupal_set_message(t('Everything failed; try again.'), 'error', FALSE);
    $form_state['redirect'] = 'admin/ret/import/user/overview';
  }
}

function _ret_user_import_import_csv($srcinfo, &$context) {
  if (!isset($context['sandbox']['max'])) {
    $context['sandbox']['max'] = (count(file($srcinfo->filepath)) - 1); // -1 because we read the first line as headers
    $context['sandbox']['done'] = $srcinfo->offset;
    $context['sandbox']['rows'] = 0;
  }

  // Return for feedback every X seconds
  $timelimit = time() + 10;

  while ($values = _ret_user_import_get_row($srcinfo->filepath, $context['sandbox']['done'])) {
    // do your work

    $context['sandbox']['rows']++;
    if (time() > $timelimit) {
      break;
    }
  }

  // provide a meaningful status message during the batch process
  $context['message'] = t('%rows of %total records imported.',
                          array('%rows' => $context['sandbox']['rows'],
                                '%total' => $context['sandbox']['max']));
  
  // if not finished, give the Batch API a percentage of 'finished' to use for the progress bar
  if ($context['sandbox']['rows'] != $context['sandbox']['max']) {
    $context['finished'] = $context['sandbox']['rows'] / $context['sandbox']['max'];
  }
  else {
    $context['finished'] = 1;
  }
  
  // if finished, wrap up
  // the _finish function below will be called after this
  if ($context['finished'] == 1) {
    $context['results']['srcinfo'] = $srcinfo;
    $context['message'] .= '<br/>' . t('Importation Complete.');
  }
}

/**
 * Batch _finish function
 *
 * This function uses the success, results and operations parts of the $context variable,
 *   executing after the batch has completed successfully.
 */
function _ret_user_import_import_finish($success, $results, $operations) {
  if ($success) {
    $srcinfo = $results['srcinfo'];
    unlink($srcinfo->filepath);
    drupal_set_message($srcinfo->success_msg);
  }
}
?>