php - CodeIgniter - Message: mkdir(): Permission denied on Ubuntu -

i'd please. have php script inside post_model constructor

$dir = fcpath . 'uploads' . directory_separator . 'posts';  if (!is_dir($dir)) {     mkdir($dir, 0755, true); }

which shows me error:

severity: warning  message: mkdir(): permission denied

the main idea project has ability create users , these users can upload images, or create folders-albums stored in uploads folder.

i've been struggling fix error last days , can't find solution. have tried code , on windows , works great, not on linux (ubuntu 14.04)

even had same problem , tried umask, worked. can this,

$old = umask(0); mkdir($dir, 0755, true); umask($old);

TSQL