While logging visitors keep in mind your site may be visited by Search Engine Crawlers too, to distinguish between Crawler and real user I have below function working for me well, although it may not be perfect or do not have complete list of all search engines be there are the most common and frequent crawlers visits the sites.

here is the code


function is_bot(){
$botlist = array("Teoma", "alexa", "froogle", "Gigabot", "inktomi",
"looksmart", "URL_Spider_SQL", "Firefly", "NationalDirectory",
"Ask Jeeves", "TECNOSEEK", "InfoSeek", "WebFindBot", "girafabot",
"crawler", "www.galaxy.com", "Googlebot", "Scooter", "Slurp",
"msnbot", "appie", "FAST", "WebBug", "Spade", "ZyBorg", "rabaz",
"Baiduspider", "Feedfetcher-Google", "TechnoratiSnoop", "Rankivabot",
"Mediapartners-Google", "Sogou web spider", "WebAlta Crawler","TweetmemeBot",
"Butterfly","Twitturls","Me.dium","Twiceler","bot","Bot");
foreach($botlist as $bot)
{
if(strpos($_SERVER['HTTP_USER_AGENT'], $bot) !== false)
return $bot;
}
return false;
}