<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <title>Add two numbers</title>
</head>
<body>
<?php
if (isset($_POST['submitted'], $_POST['num1'], $_POST['num2'])) {
  $sum = (int)$_POST['num1'] + (int)$_POST['num2'];
  $out = sprintf('%d + %d = %d', $_POST['num1'], $_POST['num2'], $sum);
  echo <<<EOD
<h1>two numbers added</h1>
<p>$out</p>
<a href="">add more numbers</a>
EOD;
} else {
  echo <<<EOD
<h1>Add two numbers</h1>
<form action="" method="post">
<p>
<input type="text" name="num1" /> +
<input type="text" name="num2" />
</p>
<input type="submit" value="add" />
<input type="hidden" name="submitted" value="true" />
</form>
EOD;
}
?>

</body>
</html>