Is it possible to block all subdomains without access to DNS entries in PHP, and what are the alternatives for achieving this?

To block all subdomains without access to DNS entries in PHP, one approach is to check the `$_SERVER['HTTP_HOST']` variable to see if it contains a subdomain. If a subdomain is found, you can redirect the user to a different page or display an error message. Another alternative is to use a wildcard SSL certificate to secure all subdomains, and then use server-side configuration to block access to specific subdomains. 

<?php
$subdomain = explode('.', $_SERVER['HTTP_HOST'])[0];

if ($subdomain !== 'www') {
    // Redirect user to a different page or display an error message
    header('Location: https://example.com');
    exit;
}
?>

Keywords

PHP subdomains DNS blocking alternatives

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