OnChangeEvent中的类别和子类别

我是php新手。我在选择选项的onchange事件中遇到问题…如果我改变将整个表单分类，而不是更改categores1的值。

onchange="myform.submit()"中onchange事件的问题。

<form name="myform" method="POST" action="getsearch.php"> 
        <table style=" border:1px solid silver" cellpadding="10px" cellspacing="0px" align="center"> 
            <tr> 
                <td colspan="3" style="background:#0066FF; color:#FFFFFF; font-size:20px">Search</td>
            </tr> 
            <tr>
            <td>Name</td> 
                <td><input type="text" name="name" size="40" /></td>
            </tr>
            <tr>
            <td>Email</td> 
                <td><input type="text" name="email" size="40" /></td>
            </tr>
            <tr>
            <td>Phone</td> 
                <td><input type="text" name="phone" size="40" /></td>
            </tr>

            <tr> 
                <td>categories</td> 
                <td><select name="first" onchange="myform.submit()">
                    <option value="">select the category</option>
                    <?php 
                            $sql="select *from first_category";
                            $res=mysql_query($sql);
                            echo $res;
                            if(mysql_num_rows($res)>0)
                            {
                                    while($row=mysql_fetch_assoc($res))
                                    {
                                        if(isset($_POST["first"]) AND $_POST["first"]==$row["first_id"])
                                        $select ="selected='selected'";
                                        else
                                        $select='';
                                        echo '<option value="'.$row["first_id"].'" '.$select.'>'.$row["category_name"].'</option>';
                                    }
                                    }
                        ?>
                </select></td> 
            </tr> 
            <tr> 
                <td>categories1</td> 
                <td><select name="second">
                        <option value="">select the second category</option>
                        <?php 
                            if(isset($_POST["first"]))
                            {
                                $sql="select *from second_category where first_id=".intval($_POST["first"]);
                                $res=mysql_query($sql);
                                echo $res;
                                if(mysql_num_rows($res)>0)
                                {
                                    while($row=mysql_fetch_assoc($res))
                                    {
                                        echo '<option value="'.$row["second_id"].'">'.$row["second_categoryname"].'</option>';
                                    }
                                }
                            }
                                ?>
                                    </select></td> 
            </tr> 
            <center><tr >
                    <td valign="middle"><input type="submit" value="Search" id="hi" name="hi" /></td> 
            </tr></center>
        </table>