No token found, although echo POST shows one

[jamminjames]

Do you have to use Twig? Because I'm trying the example without Twig here, and it's not finding the token. However, when I echo $_POST, it is showing a token, eg: [_CSRF_TOKEN] => XXXXXXX= (substituting 'XXXXXXX' for the token).

In the form, I have:

$server = $_SERVER;
$csrft = new AntiCSRF($server);
$token_html = $csrft->insertToken('', true);

On the action page, I have:

$csrf = new \ParagonIE\AntiCSRF\AntiCSRF;

if (!empty($_POST)) {
    if ($csrf->validateRequest()) {
        echo 'Valid form';
    } else {
        // Log a CSRF attack attempt
        echo 'Token is invalid';
    }
}

... and it results in 'Token is invalid'.

I am referencing the class in both the form and the action page with:

use ParagonIE\ConstantTime\{
    Base64UrlSafe,
    Binary
};
use ParagonIE\AntiCSRF\AntiCSRF;

Do I need a hidden input on the form, if not using Twig? I tried;
<input type="hidden" name="form_token" value="<?php echo $token_html; ?>">