我们可以通过HTTP_USER_AGENT来判断是否是蜘蛛，搜索引擎的蜘蛛都有自己的独特标志，下面列取了一部分。

function is_crawler() {
    $userAgent = strtolower($_SERVER['HTTP_USER_AGENT']);
    $spiders = array(
        'Googlebot', // Google 爬虫
        'Baiduspider', // 百度爬虫
        'Yahoo! Slurp', // 雅虎爬虫
        'YodaoBot', // 有道爬虫
        'msnbot' // Bing爬虫
        // 更多爬虫关键字
    );
    foreach ($spiders as $spider) {
        $spider = strtolower($spider);
        if (strpos($userAgent, $spider) !== false) {
            return true;
        }
    }
    return false;
} 

function isCrawler() {
        echo $agent= strtolower($_SERVER['HTTP_USER_AGENT']);
        if (!empty($agent)) {
                $spiderSite= array(
                        "TencentTraveler",
                        "Baiduspider+",
                        "BaiduGame",
                        "Googlebot",
                        "msnbot",
                        "Sosospider+",
                        "Sogou web spider",
                        "ia_archiver",
                        "Yahoo! Slurp",
                        "YoudaoBot",
                        "Yahoo Slurp",
                        "MSNBot",
                        "Java (Often spam bot)",
                        "BaiDuSpider",
                        "Voila",
                        "Yandex bot",
                        "BSpider",
                        "twiceler",
                        "Sogou Spider",
                        "Speedy Spider",
                        "Google AdSense",
                        "Heritrix",
                        "Python-urllib",
                        "Alexa (IA Archiver)",
                        "Ask",
                        "Exabot",
                        "Custo",
                        "OutfoxBot/YodaoBot",
                        "yacy",
                        "SurveyBot",
                        "legs",
                        "lwp-trivial",
                        "Nutch",
                        "StackRambler",
                        "The web archive (IA Archiver)",
                        "Perl tool",
                        "MJ12bot",
                        "Netcraft",
                        "MSIECrawler",
                        "WGet tools",
                        "larbin",
                        "Fish search",
                );
                foreach($spiderSite as $val) {
                        $str = strtolower($val);
                        if (strpos($agent, $str) !== false) {
                                return true;
                        }
                }
        } else {
                return false;
        }
}
if  (isCrawler()){
       echo "你好蜘蛛精！";
}
else{
     echo "你不是蜘蛛精啊！";
}