Create FTP Account and Directory Automatically Joining Site

when a user joins my site I need a function added to my [login to view URL] script that automatically creates a passive ftp account and directory for the user with the same username and password they used when joining the site. I created a sub domain for ftp use i.e. [login to view URL] Currently when each user joins my site, my [login to view URL] script creates a new directory for the user's $id variable in the 'users' directory which is in my public_html directory i.e. mkdir("users/$id", 0755);