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;
}

How much of a proof is this code? Because I heard that bot detection cannot be 100% proof.

@FarrisFahad the big problem with this code is that it does not verify the bot as legitimate.
People can just spoof the User-Agent.

Current Facebook header:
User-Agent: cortex/1.0
X-FB-HTTP-Engine: Liger
X-FB-Client-IP: True
X-FB-Server-Cluster: True

whats the point of passing $user_agent as an argument and after that overriding it with
$user_agent = strtolower($_SERVER['HTTP_USER_AGENT']);
I guess you dont need that argument.

Also why name the function smart_ip_detect_crawler(). It does not have anything to do with IP. Maybe you could name it just smart_detect_crawler().

This script is not meant to be the be-all-and-end-all of bot detection ;)

Hopefully it's helpful if you're working on your own system, and as @FinlayDaG33k mentioned... it's trivial to bypass this. It was originally used just as a metric for how much traffic was coming to a certain page was bots like GoogleBot and Bing's bot.

whats the point of passing $user_agent as an argument and after that overriding it with $user_agent = strtolower($_SERVER['HTTP_USER_AGENT']); I guess you dont need that argument.

I think he just forgot to clean that up.