Detect crawlers/bots/spiders in PHP (simple and fast)

crawler_detect.php

<?php

/**
 * Check if the given user agent string is one of a crawler, spider, or bot.
 *
 * @param string $user_agent
 *   A user agent string (e.g. Googlebot/2.1 (+http://www.google.com/bot.html))
 *
 * @return bool
 *   TRUE if the user agent is a bot, FALSE if not.
 */
function smart_ip_detect_crawler($user_agent) {
  // User lowercase string for comparison.
  $user_agent = strtolower($_SERVER['HTTP_USER_AGENT']);

  // A list of some common words used only for bots and crawlers.
  $bot_identifiers = array(
    'bot',
    'slurp',
    'crawler',
    'spider',
    'curl',
    'facebook',
    'fetch',
  );

  // See if one of the identifiers is in the UA string.
  foreach ($bot_identifiers as $identifier) {
    if (strpos($user_agent, $identifier) !== FALSE) {
      return TRUE;
    }
  }

  return FALSE;
}

Checking for google bot by reverse dns lookup: https://github.com/kalmargabor/crawler-check/blob/master/src/CrawlerCheck/CrawlerCheck.php