How to detect web crawlers with PHP ?

Hi Everyone,
"How to identify a web crawler in PHP ?", Consider the following example;

PHP Function

function isBotRobot(){

$machStr="Google|Alexa|yahoo";

$agent = @$_SERVER["HTTP_USER_AGENT"];

$url = @$_SERVER["REQUEST_URI"];

    if(preg_match($matchStr,$agent)){

        $GLOBALS['isBotRobot'] = true;

    }

}

Database Table

CREATE TABLE IF NOT EXISTS page_visits(

id int(10) unsigned NOT NULL AUTO_INCREMENT,

userAgent varchar(100) COLLATE utf8_general_ci DEFAULT NULL,

url varchar(100) COLLATE utf8_general_ci DEFAULT NULL,

regDate datetime DEFAULT NULL,

ipAdress varchar(15) CHARACTER SET utf8 DEFAULT NULL,

PRIMARY KEY (id)

) ENGINE MyISAM;

PHP Control

if(preg_match($matchStr,$agent)){

$DB =& Loader::loadClass('db');

$sql= "INSERT INTO page_visits (userAgent,url,regDate,ipAdress) VALUES (?,?,NOW(),?)";

$DB->insert($sql,array($agent,$url,$ipAdress);

@$GLOBALS['isBotRobot'] = true;

}