Mobile - detect or redirect

/*********** Detect ***********/
 
$agent = $_SERVER['HTTP_USER_AGENT'];  
 
if(preg_match('/iPhone|Android|Blackberry/i', $agent)){  
    echo "You're using a mobile device.";  
}  
 
/*********** Redirect ***********/
 
$agent = $_SERVER['HTTP_USER_AGENT'];  
 
if(preg_match('/iPhone|Android|Blackberry/i', $agent)){  
    header("Location: mobile.html");  
    exit;  
}