Untitled

<?php
$conn = mysqli_connect('localhost','root','' , 'selecttest') ;

 $query = "SELECT DISTINCT username FROM users";
 $result = mysqli_query($conn,$query);


 ?>
 <!DOCTYPE html>
 <html>
 <head>
    <title>Select</title>
 </head>
 <body>

<?php

$query1 = "SELECT * FROM users ";
$result1 = mysqli_query($conn,$query1);
    echo "<table>
    <tr>
    <th>first name</th>
    <th>last name</th>
    </tr>
    ";
while ($row = mysqli_fetch_assoc($result1)){
$first = $row['first'];
$last = $row['last'];
    echo "<tr>
        <td>$first</td>
        <td>$last</td>
          </tr>";
 }
 echo "</table>";


  $op = $_POST['op'];
  if (isset($_POST['ok'])) {
$query2 = "SELECT * FROM users WHERE username = '$op'";
$result2 = mysqli_query($conn,$query2);
    echo "<table>
    <tr>
    <th>first name</th>
    <th>last name</th>
    </tr>
    ";
while ($row = mysqli_fetch_assoc($result2)){
$first = $row['first'];
$last = $row['last'];
    echo "<tr>
        <td>$first</td>
        <td>$last</td>
          </tr>";
 }
}
 echo "</table>";
 ?>
<form method="POST" action="">
<select name="op">
<?php
while ($row = mysqli_fetch_assoc($result)) {
    $user = $row['username'];
    echo "
    <option>$user</option>
    ";
}
 ?>
</select>
 <input type="submit" name="ok">
 </form>
 </body>
 </html>