当生成的foldername已经存在时,重新创建foldername

如果我使用这段代码创建一个随机的foldername,我可以检查这个foldername是否已经存在,但是如何再次运行它创建一个新的foldername并再次检查它,依此类推

function createRandomFoldername()
{
    $chars = "abcdefghijkmnopqrstuvwxyzABCDEFGHIJKMNOPQRSTUVWXYZ1023456789";
    srand((double)microtime()*1000000);
    $i = 0;
    $folder = '' ;

    while ($i <= 10)
    {
        $num = rand() % 33;
        $tmp = substr($chars, $num, 1);
        $folder = $folder . $tmp;
        $i++;
    }

    return $folder;
}
$folder_random = createRandomFoldername();