PHP file traversal

$dir = "D:\workspace";
function list_file($dir = '.'){
    $list = scandir($dir);
    echo &#39;<ol>&#39;;
    foreach($list as $file){
        $target = "$dir/$file";
        if ( is_dir($target) && $file != &#39;.&#39; && $file != &#39;..&#39; ){
            echo "<li style=\"color:red;font-weight:bold;\">$file</li>";
            list_file($target);
        } else if ( $file != &#39;.&#39; && $file != &#39;..&#39;){
            echo "<li>$file</li>";
        }
    }
    echo &#39;</ol>&#39;;
}
list_file($dir);