Session Destroys On Page Refresh

<?php
session_start();
include("../config.php");
error_reporting(E_ALL ^ E_NOTICE);

if(isset($_POST['login']))
{
    $user=mysqli_real_escape_string($con,$_POST['username']);
    $pass=mysqli_real_escape_string($con,$_POST['password']);
    $enc_pass=md5($pass);

    $get_detail=mysqli_query($con,"SELECT * FROM `users` WHERE email='$user' AND password='$enc_pass' ");
    $check=mysqli_num_rows($get_detail);

    if($check>0)
    {
        $_SESSION['email']=$user;
        echo"<script>window.open('dashboard.php','_SELF')</script>";
    }

        else
    {
echo "Invalid Details";

}