Mostrar Informacion De PHP y Del Servidor

El codigo debajo muestra la informacion del Servidor y de PHP.

$ip_address   = $_SERVER['REMOTE_ADDR'];
$host_address = gethostbyaddr($ip_address);
$user_agent   = $_SERVER['HTTP_USER_AGENT'];
$referrer     = $_SERVER['HTTP_REFERER'];

echo "<p>IP address: " . $ip_address . "</p>" . "\n";
echo "<p>Host address: " . $host_address . "</p>" . "\n";
echo "<p>User agent: " . $user_agent . "</p>" . "\n";
echo "<p>Referrer: ";

if ($referrer == "") {
    echo "[ no referrer ]" . "</p>";
} else {
    echo "$referrer" . "</p>";
}