PHP – Rename command

Hi,
I am trying to move a file using the command below:
rename(“/wp-content/plugins/dm-custom/$file”, “$my_theme_root/$newfile”);
but I get the following error “Cannot rename a file across wrapper types”

I have checked http://php.net/manual/en/function.rename.php which says that “The wrapper used in oldname must match the wrapper used in newname”
I have tried variables of the oldname ($file) and newname ($newfile) but the same error shows.

Does anyone have a suggestion on this please? Very grateful for you help.

FYI Variables in above are shown below
$my_theme_root = get_stylesheet_directory_uri();
$file = ‘dm-custom-m.css’;
$newfile = ‘dm-custom-master.css’;