function themeInit($comment){
$comment = spam_protection_pre($comment, $post, $result);
}
function spam_protection_math(){
    $num1=rand(1,49);
    $num2=rand(1,49);
    echo "<label for=\"math\">请输入<code>$num1</code>+<code>$num2</code>的计算结果：</label>\n";
    echo "<input type=\"text\" name=\"sum\" class=\"text\" value=\"\" size=\"25\" tabindex=\"4\" style=\"width:218px\" placeholder=\"计算结果：\">\n";
    echo "<input type=\"hidden\" name=\"num1\" value=\"$num1\">\n";
    echo "<input type=\"hidden\" name=\"num2\" value=\"$num2\">";
}
function spam_protection_pre($comment, $post, $result){
    $sum=$_POST['sum'];
    switch($sum){
        case $_POST['num1']+$_POST['num2']:
        break;
        case null:
        throw new Typecho_Widget_Exception(_t('对不起: 请输入验证码。<a href="javascript:history.back(-1)">返回上一页</a>','评论失败'));
        break;
        default:
        throw new Typecho_Widget_Exception(_t('对不起: 验证码错误，请<a href="javascript:history.back(-1)">返回</a>重试。','评论失败'));
    }
    return $comment;
}

<?php spam_protection_math();?>