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

bot.detect.inc.php

<?php
/**
 * Check if a request originated from a crawler, spider, or bot.
 *
 * @param null|string $userAgent
 * @param array $identifiers
 * @return bool
 */
if (!function_exists('isBot')) {
    function isBot($userAgent = null, $identifiers = array())
    {
        $userAgent = isset($userAgent)
            ? $userAgent
            : $_SERVER['HTTP_USER_AGENT'];
        
        $identifiers = !is_array($identifiers) 
            ? array($identifiers) 
            : $identifiers;
        
        $userAgent = trim(strtolower($userAgent));

        $_identifiers = array(
            'bot',
            'slurp',
            'crawler',
            'spider',
            'curl',
            'facebook',
            'fetch'
        );
        
        $_identifiers = array_merge($_identifiers, $identifiers);

        foreach ($_identifiers as $_identifier) {
            if (strpos($userAgent, $_identifier) !== false) {
                return true;
            }
        }
        return false;
    }
}