Problem appending data in a file with fopen("ftp://...","a")

I am able to create the file when it is not already there but I got a "file exists" error when I try to fopen it in append mode the second time. I wrote this to store a log of the traffic on my site (userid in a cookie+timestamp) in a file. Don't know if i'm using the good technique but it's very frustrating. I tried with 2 different FTP servers where I have all the necessary rights. Is it possible to solve this problem or it's a feature in PHP? Dave Létourneau letournd@videotron.ca