PHP login and get user information

Question: How can I get the user information when the user successfully login?

Here is my login.php code

if($_SERVER['REQUEST_METHOD'] == 'POST') {
  $username = $_POST['username'];
  $password = $_POST['password'];
  // mysql connect function here....
  // mysql query here....
  $sql = "SELECT * FROM user_accounts WHERE username = '$username' and password = '$password'";
  $result = mysql_query($sql);
  $count = mysql_num_rows($result);


  if($count == 1) {
    session_register(username);
    session_register(password);
    header('location: index.php');
  }
  else {
    $error = "Invalid Username or Password Please Try Again";
  }

}

?>


Username : 
Password : 

and here is my index.php code

  session_start();
  if(!session_is_registered(username)){
  header("location:login.php");
  }
$username = session_is_registered(username);
  $password = session_is_registered(password);
echo $username;

  echo $password;
  ?>

Example: This is the database info.

User ID  --> 001
Username --> admin
Password --> admin

When I echo $username and $password the result for username = 1 and for password = 1. I wanted the result is username = admin and password = admin.

How can I get the username and password that logs? cause I need it to be query so I can get his/her e.g. First Name, Last Name.
I already get this but I forgot on how to do it. =)

Thank You so much for your replies. =)