Php login with mysql. Accepts anything as a successful login

What ever I type in the loginboxes, it accepts it as a successful login I simply cannot resolve this error... Am am very new to php, but hopefully I'll get this to work.

<?php
session_start();
include_once "inc/connect.php";

if($_POST['login']){
    include_once("inc/connect.php");
    $username = strip_tags($_POST['username']);
    $password = strip_tags($_POST['password']);

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

    $username = mysqli_real_escape_string($username);
    $password = mysqli_real_escape_string($password);
    $password = md5($password);

    $sql = "SELECT * FROM users WHERE username='$username' LIMIT 1";
    $query = mysqli_query($db, $sql);
    $row = mysqli_fetch_array($query);
    $id = $row['id'];
    $db_password = $row['password']; 

    if($password == $db_password) {
        $_SESSION['username'] = $username;
        $_SESSION['id'] = $id;
        header("Location: board.php");

    } else {
        echo "you didn't enter the corret detail";
    }


}

?>