filesystem security questions

I have a few questions about correctly implementing safe_mode, open_basedir and allow_url checks within an extension. Probably an easy question, but I have seen it implemented in various ways in different extensions and want to make sure i implement this correctly. allow_url checks: is there a standard function which produces an error if not allowed, or do I just handle this within the extension after testing with PG(allow_url_fopen)? safe_mode: this is done by testing both php_check_safe_mode_include_dir and php_checkuid if safe_mode is enabled, correct? open_basedir: this is just done with php_check_open_basedir? Is there any precedence of the safe_mode and open_basedir checks or does it need to pass both checks? Are there any other checks i am missing to implement filesystem security within an extension? Thanks, Rob