Can't write to file via php, just via ftp...Can anyone help?

Hello all, another newbie-question, probably... :-) And sorry for my (possibly) bad english, I'm from Germany. the problem I noticed is that I have to write to a file on the server. I created a directory where I store the files, upload a data-file into it and tried to write to it via a php-file. But it seems I don't have the right to write to it and somehow I can't change that with the ftp-programm I have ("Interarchy" on the Mac). So, does anybody know if there is a solution for this? I mean, others have to write to the server too, so there must be a way to do it... (btw, it's a linux or unix server where the pages are on). Thanks for your thoughts. Have a nice day, Stephan Huebner