if(isset($_POST['submit'])){
   $username=$_POST['username'];
   $password=$_POST['password'];
   $query = mysqli_query($conn, "SELECT * FROM mylogin WHERE username='$username' and password='$password'");
   if (mysqli_num_rows($query) != 0){
      echo "sucess" , header('Location: welcome.php');

    } else {
       echo "invalid username or password";
    }
}
?>

$database_name = "id2425621_login";
$conn = mysqli_connect($servername, $username, $password, $database_name);
if (!$conn) {
    die(mysqli_error($conn));
}
$sql = mysql_query("SELECT id, fname, lname, email FROM patients");

?>
<table border="2" style= "background-color: #84ed86; color: #761a9b; margin: 0 auto;" >
      <thead>
        <tr>
          <th>id</th>
          <th>fname</th>
          <th>lname</th>
          <th>email</th>

        </tr>
      </thead>
      <tbody>
        <?php
          while( $row = mysql_fetch_assoc( $sql ) ){
            echo
            "<tr>
              <td>{$row['id']}</td>
              <td>{$row['fname']}</td>
              <td>{$row['lname']}</td>
              <td>{$row['email']}</td>
            </tr>n";
          }

        ?>
      </tbody>
    </table>

use mysqli not mysql - mysql is not supported anymore.

$database_name = "id2425621_login";
$conn = mysqli_connect($servername, $username, $password, $database_name);
if (!$conn) {
    die(mysqli_error($conn));
}
$sql = mysqli_query($conn,"SELECT id, fname, lname, email FROM patients");

?>
<table border="2" style= "background-color: #84ed86; color: #761a9b; margin: 0 auto;" >
      <thead>
        <tr>
          <th>id</th>
          <th>fname</th>
          <th>lname</th>
          <th>email</th>

        </tr>
      </thead>
      <tbody>
        <?php
          while( $row = mysqli_fetch_assoc( $sql ) ){
            echo
            "<tr>
              <td>{$row['id']}</td>
              <td>{$row['fname']}</td>
              <td>{$row['lname']}</td>
              <td>{$row['email']}</td>
            </tr>n";
          }

        ?>
      </tbody>
    </table>