为啥我php用get和post方式都获取不到数据？？？求助

    <form action="action.php" method="post">
        <table>
            <tr>
                <td>id</td>
                <td><input type="text" name="id"></td>
            </tr>
            <tr>
                <td>name</td>
                <td><input type="text" name="name"></td>
            </tr>
            <tr>
                <td>sex</td>
                <td><input type="radio" name="sex" value="m">男</td>
                <td><input type="radio" name="sex" value="w">女</td>
            </tr>
            <tr>
                <td>class</td>
                <td><input type="text" name="class"></td>
            </tr>
            <tr>
                <td>&nbsp;</td>
                <td><input type="submit" name="提交"></td>
                <td><input type="reset" name="重置"></td>
            </tr>

        </table>
    </form>







                $id = $_POST['id'];
        $name = $_POST['name'];
        $sex = $_POST['sex'];
        $class = $_POST['class'];

        $sql = "insert into stu values('{$id}', '{$name}', '{$sex}', '{$class})'";
        $re = $pdo->exec($sql);

        if($re>0) {
            echo "<script>alert('增加成功');window.location ='index.php';</script>";
        } else {
            echo "<script>alert('增加失败');window.location = history.back();</script>";
        }

$sql = "insert into stu values('{$id}', '{$name}', '{$sex}', '{$class})'";
不能这么拼接，需要用加号去连接