Done :D

<?php
class Connect
{

public function __construct()
	{
		try
		{
			$this->db = new PDO("mysql:host=localhost;dbname=aaron_test",'aaron_test','d6G4X5S54');
		}
			catch(PDOException $e)
		{
			echo $e->getMessage();
		}
	}

public function login($name, $pass)
	{
		if(!empty($name) && !empty($pass))
		{
			$st = $this->db->prepare("SELECT * FROM users WHERE username= ? and password= ?");
			$st->bindParam(1, $name);
			$st->bindParam(2, $pass);
			$st->execute();

			if($st->rowCount() == 1)
			{
				echo "Access Granted.";
			}
			else
			{
				echo "incorrect username or password.";
			}
		}
		else
		{
			echo "Please supply a username and password.";
		}
	}
public function register($name, $pass)
	{
		if(!empty($name) && !empty($pass))
		{
			$get = $this->db->prepare("SELECT * FROM users WHERE username = ?");
			$get->bindParam(1, $name);
			$get->execute();

			if($get->rowCount() == 0)
			{
				$set = $this->db->prepare("INSERT INTO users (username, password) VALUES (?, ?)");
				$set->bindParam(1, $name);
				$set->bindParam(2, $pass);
				$set->execute();

				echo "Your details have now been registered.";
			}
			else
			{
				echo "An account with this username already exists.";
			}
		}
		else
		{
			echo "Please fill in all of the fields.";
		}
	}
}

?>

///// INDEX.PHP BELOW /////

<?php
require 'user.php';

if(isset($_POST['submit']))
{
	$name = $_POST['user'];
	$pass = $_POST['password'];

	$object = new Connect();
	$object->login($name, $pass);
}

if(isset($_POST['register']))
{
	$name = $_POST['reg_user'];
	$pass = $_POST['reg_pass'];

	$reg = new Connect();
	$reg->register($name, $pass);
}
?>

<!-- LOGIN STARTS -->
<h2>Login</h2>
 <form action="" method="POST">
 	Username: <input type="text" name="user">
 	Password: <input type="text" name="password">
 	<input type="submit" name="submit" value="Login">
 </form>

<!-- REGISTER STARTS -->
<h2>Register</h2>
 <form action="" method="POST">
 	Username: <input type="text" name="reg_user">
 	Password: <input type="text" name="reg_pass">
 	<input type="submit" name="register" value="Register">
 </form>