php查询数据库有没有字段相等

<!--登录界面-->
<form action="3.php" method="post"><!--表单提交action到chuli界面，提交方式设置为post。-->

<div>用户名：<input type="text"  name="uid"/></div> <!--用户名输入框-->

<div><input type="submit" value="登录" /></div>

</form>

$uid = $_POST["uid"];

$db = new MySQLi("localhost","root","123.cn","123.cn");
mysqli_connect_error()?die("连接失败"):"";
$sql = "select count(*) from dede_admin where userid=‘{$uid}‘";

$result = $db->query($sql);

$n = $result->fetch_row();

if($uid !="")
{
    if($n[0]==1)
    {
        header("location:main.php");
    }
    else
    {
        echo "<script>alert(‘不正确‘);window.history.go(-1);</script>";
    }
}
else{
    
    echo "<script>alert(‘不能为空‘);window.history.go(-1);</script>";
}