Issue with array_filter

Question

$files = array_filter(scandir($dir), function($item) {
    return !is_dir('../folder/pictures/3/' . $item);
});

echo("<pre>".print_r($files,true)."</pre>");

I need to scan a directory but I only need file names and not folders. The code above works great, however when I use it in this way it stops working?

$dir    = "..".$config[picurl]."/".$pic_id;

$files = array_filter(scandir($dir), function($item) {
    return !is_dir($dir .'/'. $item);
});

Why can't I use the variable $dir?