Have a look into is_dir() and is_file()
<?php
$path = "C:\\test_folder\\folder2\\folder4";
$sub_folder = scandir($path);
$num = count($sub_folder);
for ($i = 2; $i < $num; $i++)
{
if(is_file($path.'\\'.$sub_folder[$i])){
echo 'Warning';
}
}
?>