There is a bug in my first message:
$uploadVars['url'] = 'http://upload.wikimedia.org/wikipedia/en/b/bc/Wiki.png';
//$uploadVars['file'] = $fileContent;

You must not use both params. I proved but it isn't still working. I have another problem: I installed LAMP and there is no support to curl by default, so I might have to compile from source...
I still think that code must work with minimal modifications. I suspect the problem is the wrapper: According with wikimedia doc, it must be a multipart/form-data request, and I'm not sure if I'm doing the last step correctly.