Bad Code

<?php

include 'db_connection.php'; // Assume this external script connects us to the database

// Start session
session_start();

// Extract POST variables: 'username', 'password', and 'filename'
extract($_POST);

// Check if user is already logged in
if ($_SESSION['is_logged_in'] == '1') {

	// Output confidential file contents to user
	echo shell_exec('cat ~/confidential_info/' . $filename);

} else {

	// Check login credentials
	$query =
		"SELECT * FROM users WHERE ".
		"username = '" . $username . "' AND ".
		"password = '" . $password . "';"
	$result = mysql_query($query);

	if (mysql_num_rows($result) > 0) {

		// Login information matched credentials!
		$_SESSION['is_logged_in'] = '1';

		// Output confidential file contents to user
		echo shell_exec('cat ~/confidential_info/' . $filename);

	} else {

		// Login information did not match!
		die('You do not have access.');

	}
}

?>