Your file paths should be relative to their location in the file system. Where in the file system is the PHP file doing the writing compared to where you're trying to create the file. It should be irrelevant whether it's a different subdomain or not.
sorry been busy fixing other site issues but its not working as soon as i get to ../../../https within the sub domain its ok but when i go one more the next dir name is mobile it starts throwing Warning: is_dir() [function.is-dir]: open_basedir restriction in effect. File(../../../../mobile) is not within the allowed path(s): mobile is the subdomain name before mobile is dir subdomains