Undefined Variable Before Submit IS USE

<?php
$servername = "localhost";
$username = "root";
$password = "";
$database = "abc";

$dblink = mysqli_connect($servername,$username,$password,$database);

if(isset($_POST['submit'])) {
  if($_POST['select']=='all_records') {
    $query = "SELECT Kod_Dorm
        FROM dorm
        ";
  }
elseif($_POST['select']=='A302') {
    $query = "SELECT Kod_Dorm
            FROM dorm
            WHERE Kod_Dorm = 'A302'";
  }
    elseif($_POST['select']=='A303') {
    $query = "SELECT Kod_Dorm
            FROM dorm
            WHERE Kod_Dorm = 'A303'";
  }

$results = mysqli_query($dblink, $query) or die (mysqli_error());
}

echo "<table>"; // start a table tag in the HTML

while($row = mysqli_fetch_array($results)){   //Creates a loop to loop through results
echo "<tr><td>".$row['Kod_Dorm']."</td></tr>";  //$row['index'] the index here is a field name
}

echo "</table>"; //Close the table in HTML

?>

<form class="filteroption" action="" method="post">
          <select class="select" name="select">
            <option value ="all_records" selected="selected">All  records</option>
            <option value ="A302" >A302</option>
            <option value ="A303" >A303</option>
          </select>
            <input  class="" type="submit" name="submit" value="submit">
          </form>