If you really need to detect GOOGLE engine bots you should never rely on "user_agent" or "IP" address because "user_agent" can be changed and acording to what google said in: Verifying Googlebot
To verify Googlebot as the caller:
1.Run a reverse DNS lookup on the accessing IP address from your logs, using the host command.
2.Verify that the domain name is in either googlebot.com or google.com
3.Run a forward DNS lookup on the domain name retrieved in step 1 using the host command on the retrieved domain name. Verify that it is the same as the original accessing IP address from your logs.
Here is my tested code :
<?php
$remote_add=$_SERVER['REMOTE_ADDR'];
$hostname = gethostbyaddr($remote_add);
$googlebot = 'googlebot.com';
$google = 'google.com';
if (stripos(strrev($hostname), strrev($googlebot)) === 0 or stripos(strrev($hostname),strrev($google)) === 0 )
{
//add your code
}
?>
In this code we check "hostname" which should contain "googlebot.com" or "google.com" at the end of "hostname" which is really important to check exact domain not subdomain. I hope you enjoy ;)