How To Prevent DDOS Attack By PHP

According the PHP part of the question;
Although I don't rely on PHP for this, it could be implemented but needs to consider all these
possiblities or more;

Attacker may change IP for each request
Attacker may pass parameter(s) to URI that target site doesn't care these parameter(s)
Attacker may restart the session before expiry ...
Simple pseudo;

<?php                                                                        
// Assuming session is already started                      
$uri = md5($_SERVER['REQUEST_URI']);          
$exp = 3; // 3 seconds                                              
$hash = $uri .'|'. time();                                            
if (!isset($_SESSION['ddos'])) {                              
    $_SESSION['ddos'] = $hash;                              
                                                                                   
}                                                                              
                                                                                   
list($_uri, $_exp) = explode('|', $_SESSION['ddos']);
if ($_uri == $uri && time() - $_exp < $exp) {
    header('HTTP/1.1 503 Service Unavailable');
    // die('Easy!');
    die;
}

// Save last request
$_SESSION['ddos'] = $hash;
?>

BY qeremy
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