PHP无限级分类的展示有关问题。折腾一天了，搞不定呀

<br />//无限分类显示函数<br />function get_str($id = 0) { <br />    global $str; <br />    $sql = "select classid,classname,classstate,classtype,classurl,classpage,classorder,classtitle,classkeyword,classdescription,classpid from bz_class where classpid= $id";  <br />    $result = mysql_query($sql);//查询pid的子类的分类 <br />    if($result && mysql_affected_rows()){//如果有子类 <br />        while ($row = mysql_fetch_array($result)) { //循环记录集<br />		 	$str.= "<tr><td>" .$row['classid']. "</td><td>". $row['classname']. "<p>id: 882233</p>
                    <div class="aritcle_card">
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                            <img src="https://img.php.cn/upload/ai_manual/001/503/042/68b6cb08a59ca496.jpeg" alt="Voicepods">
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                            <a href="/ai/1051">Voicepods</a>
                            <p>Voicepods是一个在线文本转语音平台，允许用户在30秒内将任何书面文本转换为音频文件。</p>
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                                <img src="/static/images/card_xiazai.png" alt="Voicepods">
                                <span>93</span>
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                        <a href="/ai/1051" class="aritcle_card_btn">
                            <span>查看详情</span>
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                </td><td>". $row['classtype']."</td><td>". $row['classid']."</td><td>". $row['classid']."</td><td>". $row['classid']."</td></tr>" ; //字符串构建<br />            get_str($row['classid']); //调用get_str()，将记录集中的id参数传入函数中，继续查询下级 <br />        } <br />    } <br />    return $str; <br />} <br />