php查询

<h1>查询数据</h1>
<form action="chaxun.php" method="post">       <!--建立表单 action 查询后返回本页面显示结果-->
<div>请输入：<input type="text" name="xingming" /> <input type="submit" value="查询" /></div>         <!--name是服务端用的，定义个名称-->
<br />
</form>
<table width="100%" border="1" cellpadding="0" cellspacing="0">
    <tr>
        <td>学号</td>
        <td>姓名</td>
        <td>性别</td>
        <td>班级</td>
       
    </tr>
    <?php
    $tj = " 1=1 "; //做了一个恒成立的条件     
    $xingming = "";
    if(!empty($_POST)){                         //判断值是否为空
        $xingming = $_POST["xingming"];
        $tj = " xingming like '%{$xingming}%' ";             //用like建立模糊查询
    }
    
    $db = new MySQLi("localhost","root","5270437","ceshi");     
    $sql = "select * from xueshengbiao where {$tj}";      //查找表加条件
    echo $sql;              //输出一下看看是否连接
    $relsut = $db->query($sql);
    if($relsut){
        $arr = $relsut->fetch_all();
        foreach($arr as $v){
            $v[1]; $xingming;
            $carname = str_replace($xingming,"<span style='color:red'>{$xingming}</span>",$v[1]);
            echo "<tr>
        <td>{$v[0]}</td>
        <td>{$carname}</td>
        <td>{$v[2]}</td>
        <td>{$v[4]}</td>
    </tr>";
        }
    }
    ?>
</table>