PHP and MySQL - modify records

<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();
}
?>