$email = mysqli_real_escape_string($con,$_POST['user']);
$pass = mysqli_real_escape_string($con,$_POST['pass']);
$sel_user = "select * from admins where email='$email' AND password='$pass'";
$run_user = mysqli_query($con, $sel_user);
$check_user = mysqli_num_rows($run_user);
if($check_user>0)
{
$_SESSION['email']=$email
echo "<script>window.open('index2.html','_self')</script>";
}
else {
echo "<script>alert('Email or password is not correct, try again!')</script>";
}

<?php
       session_start();
?>
    <div class="inline-block">
                                <h5 class="no-margin"> Welcome </h5>
                                <h4 class="no-margin"> <?php echo $_SESSION['email']; ?> </h4>
                                <a class="btn user-options sb_toggle">
                                    <i class="fa fa-cog"></i>
                                </a>
                            </div>