php file_put_contents function not working

It could be a permission problem

Is the directory /files chmoded to 777? sometimes php won't allow you to access directories if they don't have enough permission. I don't know about blank errors though.

Try to see if it has enough permissions, if not, then set it to 777 and try it.

Are you using the full path on the filesystem or are you trying to use the URI? I think this PHP function expects you to give the path as the file is found on the filesystem.

Relative paths should be okay though.

You may need to make sure the file exists and that it's permissions are set to 777. Sometimes I've found that it's not enough to just have the directory permissions set to 777 but the file must also already exist.