[development] creating a file - path problems

I've run into a problem that I can't resolve with the docs because there's no usage examples (I know, I know...but I can't write it if I don't know it). D6. I need to create a file. Seems simple. But I can't determine what combination of variables give me a good path. $file_directory_path gives me sites/default/files which is great if I have something to append it to, such as /users/myusername/public_html/mydomain.com/drupal but in looking at the descriptions of the various file api functions/vars, I don't seem to see one that does that as opposed to checking this, verifying that, etc.