PHP Query Database There are no fields equal

Index.html

<!--Login Interface -<formAction= "3.php"Method= "POST"><!--The form submits the action to the Chuli interface, and the Submit method is set to post.  -<Div>User name:<inputtype= "text"name= "UID"/></Div> <!--User name Input box -<Div><inputtype= "Submit"value= "Login" /></Div></form>

3.php

$uid=$_post["UID"];$db=NewMysqli ("localhost", "root", "123.cn", "123.cn");Mysqli_connect_error()? die("Connection Failed"): "";$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 (' incorrect '); Window.history.go ( -1);</script>"; }}Else{        Echo"<script>alert (' cannot be empty '); Window.history.go ( -1);</script>";}

PHP Query Database There are no fields equal