No Database Selected error on php page

Hi,

I am getting "No Database selected" eroor,Here's the code:

<?php
	$Username = $_POST['username'];
	$Password = $_POST['password'];
	
	
	// 3020ta, 3020
	if (($Username != '3020ta' AND $Password != '3020') && ($Username != 'asdf' AND $Password != '') && 
			($Username != '1000' AND $Password != '2000') &&  ($Username != '1100' AND $Password != '1200')&& 
			($Username != '1111' AND $Password != '2222'))
	{
		//echo $error_message
	
		echo '<a href = "Login.html"> Back To Login Page</a>';
	}
	else
	{
	require 'database.php';
	
	$sql = "SELECT * FROM akkiran";
	$result = mysql_query($sql) or die(mysql_error());
	echo "<table border='1'>";
	echo "<tr><th> Crt.no.</th> <th>Student id</th> <th>Password</th> </tr>";
	while($row = mysql_fetch_array($result))
	{
		echo "<tr><td>"; 
		echo $row['Crt.no.'];
		echo "</td><td>"; 
		echo $row['student_id'];
		echo "</td><td>"; 
		echo $row['password'];
		echo "</td></tr>"; 
	 
	}
}
	 echo "</table>";
	
?>

and here's the code for database.php:

<?php
	$dsn = 'mysql:host=127.0.0.1;dbname=ITEC3020';
	$username = 'hhhhhh';
    $password = '123456';
	
	try
	{
		$db= new PDO($dsn, $username, $password);
	}
	catch (PDOException $e)
	{
		$error_message = $e->getMessage();
		include('database_error.php');
		exit();
	}
?>

When I don't use require 'database.php' and
use

$connect = mysql_connect("127.0.0.1", "hhhhhh", "123456");
mysql_select_db("ITEC3020");

it works!!.. But I have to use require database.php..Where am I going wrong?..
Any help would be appreciated.
Thank You