PHP Intercept robot

<?php

function getrobot() {
    if(!defined('IS_ROBOT')) {
        $kw_spiders = 'Bot|Crawl|Spider|slurp|sohu-search|lycos|robozilla';
        $kw_browsers = 'MSIE|Netscape|Opera|Konqueror|Mozilla';
        if(!strexists($_SERVER['HTTP_USER_AGENT'], 'http://') && preg_match("/($kw_browsers)/i", $_SERVER['HTTP_USER_AGENT'])) {
            define('IS_ROBOT', FALSE);
        } elseif(preg_match("/($kw_spiders)/i", $_SERVER['HTTP_USER_AGENT'])) {
            define('IS_ROBOT', TRUE);
        } else {
            define('IS_ROBOT', FALSE);
        }
    }
    return IS_ROBOT;
}
getrobot();
if(defined('NOROBOT') && IS_ROBOT) {
    exit(header("HTTP/1.1 403 Forbidden"));
}


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