drupal/core/modules/aggregator/aggregator.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the aggregator module.
 */
use Drupal\Core\Language\Language;

/**
 * Implements hook_requirements().
 */
function aggregator_requirements($phase) {
  $has_curl = function_exists('curl_init');
  $requirements = array();
  $requirements['curl'] = array(
    'title' => t('cURL'),
    'value' => $has_curl ? t('Enabled') : t('Not found'),
  );
  if (!$has_curl) {
    $requirements['curl']['severity'] = REQUIREMENT_ERROR;
    $requirements['curl']['description'] = t('The Aggregator module could not be installed because the PHP <a href="@curl_url">cURL</a> library is not available.', array('@curl_url' => 'http://php.net/manual/curl.setup.php'));
  }
  return $requirements;
}