<?php

$directory = "./01/";
$namePattern = "_o.jpg"
$miniaturePattern = "_o-lightbox.jpg";

$images = glob($directory . "*".$namePattern);

foreach($images as $image):
?>
    <li>
        <a href="<?php echo $image; ?>" class="html5lightbox">
            <img src="<?php echo str_replace($namePattern, $miniaturePattern, $image); ?>"/>
        </a>
    </li>
<?php endforeach; ?>