[development] creating a file - path problems

[Jeff Greenberg]

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.