$array1 = range('0','9');
    $array2 = range('a','f');
    $dirstrings = array_merge($array1,$array2);

    foreach($dirstrings as $dir) {
      foreach($dirstrings as $subdir) {
        system("mkdir -p " . $path . "/{90x90,45x45,full}/" . $dir . "/" . $subdir . "/");
      }
    }

// Replacement

        for($dir = 0; $dir < 16; $dir++)
            for($subdir = 0; $subdir < 16; $subdir++){
                $complete_path = $path . '/{90x90,45x45,full}/' . sprintf("%01x/%01x", $dir, $subdir) . '/';
                if(! mkdir($complete_path, 0755, true))
                    return false;
            }