如何在没有插件的情况下将验证码添加到WordPress管理员登录表单

// 生成验证码
function generate_captcha() {
    $captcha = rand(1000, 9999);
    $_SESSION['captcha'] = $captcha;

    $image = imagecreatetruecolor(100, 30);
    $bg_color = imagecolorallocate($image, 255, 255, 255);
    $text_color = imagecolorallocate($image, 0, 0, 0);

    imagefilledrectangle($image, 0, 0, 100, 30, $bg_color);
    imagettftext($image, 20, 0, 10, 25, $text_color, 'path/to/font.ttf', $captcha);

    header('Content-type: image/png');
    imagepng($image);
    imagedestroy($image);
}

// 添加验证码字段到登录表单
function add_captcha_field() {
    echo '<p>';
    echo '<label for="captcha">验证码:</label>';
    echo '<input type="text" name="captcha" id="captcha" required>';
    echo '</p>';
}

// 验证验证码
function validate_captcha($user, $username, $password) {
    if (isset($_POST['captcha']) && $_POST['captcha'] != $_SESSION['captcha']) {
        $user = new WP_Error('denied', "验证码错误，请重试。");
    }
    return $user;
}

// 在登录表单中添加验证码字段
add_action('login_form', 'add_captcha_field');

// 验证验证码
add_filter('authenticate', 'validate_captcha', 30, 3);