php mysqlでレコードを変更する方法

<h1>修改记录</h1> 
<?php 
    if(isset($_POST[&#39;btnModify&#39;])){ 
        //验证表单省略 
        $sql = "UPDATE animal SET WHERE ID = &#39;$_POST[id]&#39;"; 
        $result = mysqli_query($sql);   //执行更新 
        if($result){ 
            echo "修改已经成功！"; 
        } 
        else 
        { 
            echo "修改失败！"; 
        } 
    } 
    //查询当前的记录 
    $query = "SELECT * FROM symbols"; 
    //执行该查询 
    if($result = $mysqli->query($query)){ 
        //显示返回的记录集行数 
        if($result->num_rows>0){ 
            //如果有记录 
            //显示记录集中列的内容 
            echo "<table cellpadding=10 border=1>"; 
            while($row=$result->fetch_array()){ 
                echo "<tr>"; 
                echo "<td><input name=&#39;id&#39; type=&#39;text&#39; id=&#39;id&#39; value=&#39;$row[0]&#39;/></td>"; 
                echo "<td><input name=&#39;country&#39; type=&#39;text&#39; id=&#39;country&#39; value=&#39;$row[1]&#39;/></td>"; 
                echo "<td><input name=&#39;animal&#39; type=&#39;text&#39; id=&#39;animal&#39; value=&#39;$row[2]&#39; /></td>"; 
                echo "<td><input name=&#39;btnModify&#39; type=&#39;submit&#39; id=&#39;btnModify&#39; value=&#39;修改&#39; /></td>"; 
                echo "</tr>"; 
            } 
        } 
        //释放对象所占用的内存 
        $result->close(); 
    } 
?>