if(isset($_POST['submit'])) {
    $student_name = mysqli_real_escape_string($db, $_POST['student_name']);
    $update = "UPDATE students SET student_name = $student_name WHERE email = ".$_SESSION['email']."";
    $query1 = mysqli_query($db,$update);

    if($query1 !== FALSE) {
        echo 'update successfully';
        print_r($query1);
    } else {
        echo 'update not successfully';
        print_r($query1);
    }

}