<?php

if (isset($_POST['submit'])) {

echo "$suma";

}



$rub=_string($_POST['rub']);
$uah=_string($_POST['uah']);

$suma= $rub+$uah;





echo '</br>
<form action="" method="POST">
<input type="text" name="rub" /><input type="text" name="uah" />
<input type="submit" name="submit" value="Сумма" />';


?>