Message: mkdir(): Permission denied

I have this PHP script inside my Post_model constructor:

PHP Code:

$dir = FCPATH . 'uploads' . DIRECTORY_SEPARATOR . 'posts'; 
if (!is_dir($dir)) {
    mkdir($dir, 755, true);
} 


which shows me this error:

Code:

Severity: Warning

Message: mkdir(): Permission denied

and can't create the posts folder in order to upload images. If I comment this code and create the uploads/posts folder manually and then try to upload a file, the upload library shows my an error saying that the folder is not writable. How can I fix this error ?