What potential security implications should be considered when accessing URLs without "www" in PHP?

When accessing URLs without "www" in PHP, potential security implications to consider include the possibility of cookie leakage if cookies are not properly configured to be domain-specific, potential redirection vulnerabilities if the URL is not properly validated, and the risk of phishing attacks if users are redirected to malicious websites. To mitigate these risks, ensure that cookies are set with the appropriate domain attribute, validate and sanitize user input to prevent redirection to unauthorized URLs, and use HTTPS to prevent man-in-the-middle attacks.

// Set cookies with domain attribute to prevent cookie leakage
setcookie("cookie_name", "cookie_value", time() + 3600, "/", "example.com", true, true);

// Validate and sanitize URL input to prevent redirection vulnerabilities
$url = filter_var($_GET['url'], FILTER_SANITIZE_URL);
if (filter_var($url, FILTER_VALIDATE_URL)) {
    header("Location: " . $url);
    exit();
}

// Use HTTPS to prevent man-in-the-middle attacks
if ($_SERVER['HTTPS'] !== 'on') {
    header('Location: https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
    exit();
}