<?php if ( ! isset($_POST['item_a'], $_POST['item_b'])): ?>
<!-- Begin form -->
<form action="" method="POST" role="form">
<!-- Item a -->
<div class="from-gorup">
<label for="item_a">Enter item_a</label>
<input type="text" name="item_a" class="form-control" id="item_a">
</div>
<!-- Item b -->
<div class="from-gorup">
<label for="item_b">Enter item_b</label>
<input type="text" name="item_b" class="form-control" id="item_b">
</div>
<!-- Submit button -->
<button type="submit" class="btn btn-primary">Submit!</button>
</form><!-- End form -->
<?php else: ?>
<p>item_a + item_b = <?php echo $_POST['item_a'] + $_POST['item_b']; ?></p>
<?php endif; ?>