<?php
if (isset($_POST['ok'])) {
$a = isset($_POST['a']) ? abs($_POST['a']) : false;
$b = isset($_POST['b']) ? abs($_POST['b']) : false;
 
echo 'Natija:' . $a + $b;
}
?>
 
<form method="post">
<input type="text"  name="a"><br>
<input type="text" name="b"><br>
<input type="submit" name="ok" value="Qo'shish">
</form>