C++ and PHP CGI

How can I use php to submit a value to a c++ program and then receive the result and display it. Here's an example of the php I am trying to use:

<?php
INCLUDE 'index.php'; 
$form1 = $_POST['input1']; //get data from html form and store in php variable
exec("http://addresstoprogram/program $form1",$output); //attempt to use exec to execute external program and receive the result
echo $output;

?>