if(!empty($_POST) && $_POST['Action']=='login_form'){
    /* Define return | here result is used to return user data and error for error message */
    $Return = array('result'=>array(), 'error'=>'');

    $email = safe_input($con, $_POST['Email']);
    $password = safe_input($con, $_POST['Password']);

    /* Server side PHP input validation */
    if(filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
        $Return['error'] = "Please enter a valid Email address.";
    }elseif($password===''){
        $Return['error'] = "Please enter Password.";
    }
    if($Return['error']!=''){
        output($Return);
    }

    /* Check Email and Password existence in DB */
$result = mysqli_query($con, "SELECT * FROM user WHERE username='$email' AND password='MD5($password)");
    if(mysqli_num_rows($result)==1){
        $row = mysqli_fetch_assoc($result);
        /* Success: Set session variables and redirect to Protected page */
        $Return['result'] = $_SESSION['UserData'] = array('user_id'=>$row['user_id']);
    } else {
        /* Unsuccessful attempt: Set error message */
        $Return['error'] = 'Invalid Login Credential.';
    }
    /*Return*/
    output($Return);
}