Here is a recursive function that uses is_dir(), to make a list of all subdirectories inside a given directory. It returns the result in an array.

function retrieveDirs($rootdirpath) {
if ($dir = @opendir($rootdirpath)) {
$array[] = $rootdirpath;
while (($file = readdir($dir)) !== false) {
if (is_dir($rootdirpath."/".$file) && $file != "." && $file != "..") {
$array = array_merge($array, retrieveDirs($rootdirpath."/".$file));
}
}
closedir($dir);
}
return $array;