It would be great if this plugin allows us put php code in both ways direct php code and with shortcode too,

Here is the way you can put php code into visual editor.

Directly

1. User can simply put code like this

<?php 
    $a  = 1;
    $b = 2;
    echo $a+b;
?>

Because visual editor change '<' and '>' character to < and > therefore we can allow user to put php code with shortcode too. Below is the example how can you add code through shortcode without using any plugin.

using Shortcode

[vphp]
    $a  = 1;
    $b = 2;
    echo $a+b;
[/vphp]