setting up FTP account names via PHP

Hello everybody, I am running a real estate site where I would like to enable bulk upload via real estate software that exports an xml file into an ftp account. In order to give every user unique access I would need to generate individual ftp name and passwords for each member. I can not see how this should work. My portal is written in PHP 4.x and there every members loges in with a unique ID. The FTP Server runns on the same linux machine. How could I generate users for this FTP server with php, for example on sign up? Thank you for any help on this. Best regards, Merlin