Something

<?php
// --------------------------
// fun.php - Log user agents
// --------------------------
// just include it into your
// page and call the function
// --------------------------
// example:
//      include('fun.php');
//      sactivity();
// --------------------------
// have fun :)
// --------------------------

// config here
// 'string to look for' => 'Name to write into log',
$users = array(               // array of user agents and names to search for.
 'googlebot'=>'Google Bot',
 // add new ones between here

 // and here  - keep the comma on the end
 'yahoo'=>'Yahoo Bot'
);
$logfile = "log.txt";         // the log file

// end config


function sactivity (){ // the user-agent checking function

global  $users;  // array is outside the function, so declair it as global
            foreach($users as $search=>$name) {                                 // start the loop through the array
                if(strstr(strtolower($_SERVER['HTTP_USER_AGENT']),$search)) {   // is the search string in the user agent?
                    logit("$name activity found");                              // yep, lets log it!
            }
    }                                                                           // next...
}

function logit($text) { // simple function to log the time and some text.

    global $logfile;
    $h = fopen("$logfile","a");
    fputs($h,date("r")." -> $text");
    fclose($h);

}

?>