Sources: <a href="./inc_test.phps">inc_test.php</a> | <a
href="./a.phps">a.php</a> | <a
href="./b.phps">b.php</a>
<br /><hr /><br />
<?php

ini_set('display_errors', 'On');

echo 'Prior to include ... <br /><br />';

$variable = !isset($_GET['b']);

if ($variable) {
    echo 'Including a.php<br />';
    include 'a.php';
} else {
    echo 'Including b.php<br />';
    include 'b.php';
}

echo 'After include ... <br /><br />';

?>