$fullname = '';
if (isset($_POST['fullname'])) $fullname = validateFullName($_POST['fullname']);
<label for="fullname">Full Name
<span class="small">First Name Last Name</span>
<input type="text" value="<?php echo $fullname ?>" name="fullname" id="fullname"/>
If you are serving the (X)HTML5 as XHTML then you need the closing / but if you are serving it as HTML then you don't.