basic php mysql login script without page reload

insert.php

<?php
    define('HOST','localhost');
    define('USERNAME', 'root');
    define('PASSWORD','');
    define('DB','mydba');
    session_start();
    $con = mysqli_connect(HOST,USERNAME,PASSWORD,DB);
   
    $username = $_POST['username'];
    $pass = $_POST['password'];
   
    //$_SESSION["username"] = $username;
   
   
   
    //$sql = "select * from users where username='$username' && password='$pass'";
   
    //if(mysqli_query($con, $sql)){
        //echo 'success';
        //echo "   username is " . $_SESSION["username"] . ".";
    //}
   
    $sql="SELECT * FROM users WHERE username='$username' and password='$pass'";
    $result=mysqli_query($con, $sql);
    $count=mysqli_num_rows($result);
    if($count==1){
    //session_register("username");
    //session_register("pass");

    $_SESSION = mysqli_fetch_array($result, MYSQLI_ASSOC);
   
    header("Location: page5.php");         

        }else
        {
           
            echo 'Either Your Account is inactive or Email address /Password is Incorrect';
        }
   
   
?>


<?php
//session_start();
$_SESSION['id']="";
?>
<div id="loginbutton">login button</div>

<div id="menu">
<ul>
<li>menu1</li>
<li>menu2</li>
<li>menu3</li>   
</ul>
</div>
<?php
if (!isset($_SESSION['id']) || empty($_SESSION['id'])){  ?>

<script>
document.getElementById("menu").style.display = "none";
document.getElementById("loginbutton").style.display = "show";
</script>
<?php }
else {  ?>
<script>
document.getElementById("loginbutton").style.display = "none";
document.getElementById("menu").style.display = "show";
</script>

<?php
}
?>


insert.js
$('#myform').submit(function(){
    return false;
});

$('#insert').click(function(){
    $.post(       
        $('#myform').attr('action'),
        $('#myform :input').serializeArray(),
        function(result){
            $('#result').html(result);
        }
    );
});