PHP - PHP Warning: Cannot modify header information - headers already sent by

<?php

   include('dbcon.php');

   $username = $_POST['username'];

   $password = $_POST['password'];

  $query = $conn->query("select * from user where username = '$username' and password = '$password'");

  $count = $query->rowcount();

  $row = $query->fetch();

    if ($count > 0){

    session_start();

    $_SESSION['id'] = $row['id_user'];

     header('location: ../home.php');

    }else{

     echo '

		<script>

		alert("El usuario o contraseña son incorrectos. Intentelo de nuevo");

		window.location = "../login.php";

		</script>

		';

		session_destroy();

		die();

   }

?>

<?php

session_start();

include('dbcon.php');

$username = $_POST['username'];

$password = $_POST['password'];

...

?>

<?php $conn = new PDO('mysql:host=ip;dbname=db_name', 'admin', 'x'); ?>