自定义注册表单和验证码验证

Question

如何在自定义注册表单中获得简单的验证码验证？ 我有简单代码的形式和行：

<?php 
$factor1 = rand (1,10);
$factor2 = rand(1,10);

$sum = $factor1 + $factor2;

$wp_session = WP_Session::get_instance(  );
$wp_session['favcolor'] = $sum;
?>

<div class="bbp-captcha">
    <label for="captcha">Match</label>
    <input type="text" name="captcha" value="&nbsp;&nbsp;<?php echo $factor1; ?>&nbsp;&nbsp;+&nbsp;&nbsp;<?php echo $factor2; ?>" disabled />
</div>

<div class="bbp-respond">
    <label for="respond">Result</label>
    <input type="number" name="result" required />
</div>

在我的functions.php中，我添加了：

add_filter( 'register_post', 'set_user_register', 10, 3 );
function set_user_register($sanitized_user_login, $user_email, $errors) {

    $wp_session = WP_Session::get_instance(  ) ;

    if ( $_POST['result'] !== $wp_session['favcolor'] ) {
        $errors->add( 'not_human', "<strong>ERROR</strong>: Are you bot?" );
    }
}

我唯一得到的是error (not_human)。