I need help with figuring out what I am doing wrong with my login page?

<?php
session_start();

require("connect_db.php");
include('login_header.php');

if($_POST['login_btn']) {
    $username = strip_tags($_POST['username']);
    $password = strip_tags($_POST['password']);

    $username = stripslashes($username);
    $password = stripslashes($password);

    $username = mysql_real_escape_string($username);
    $password = mysql_real_escape_string($password);

    $result = mysql_query("SELECT * FROM client WHERE username = '$username' AND password = '$password'");
    $row = mysql_fetch_assoc($result);
    if($row['username'] == $username && $row['password'] == $password) {
        echo 'logged in';
    } else {
        echo 'Username/Password invalid!';
    }
}
?>