Listagem de diretórios com PHP

<?php
$root = __DIR__;

function is_in_dir($file, $directory, $recursive = true, $limit = 1000) {
    $directory = realpath($directory);
    $parent = realpath($file);
    $i = 0;
    while ($parent) {
        if ($directory == $parent) return true;
        if ($parent == dirname($parent) || !$recursive) break;
        $parent = dirname($parent);
    }
    return false;
}

$path = null;
if (isset($_GET['file'])) {
    $path = $_GET['file'];
    if (!is_in_dir($_GET['file'], $root)) {
        $path = null;
    } else {
        $path = '/'.$path;
    }
}

if (is_file($root.$path)) {
    readfile($root.$path);
    return;
}

if ($path) echo '<a href="?file='.urlencode(substr(dirname($root.$path), strlen($root) + 1)).'">..</a><br />';
foreach (glob($root.$path.'/*') as $file) {
    $file = realpath($file);
    $link = substr($file, strlen($root) + 1);
    echo '<a href="?file='.urlencode($link).'">'.basename($file).'</a><br />';
}
?>

Fonte: http://stackoverflow.com/questions/6223365/building-a-simple-directory-browser-using-php-recursivedirectoryiterator