User permissions

I've installed apache2 on debian with mod_vhost. In [...]/htdocs/ are
symlinks like www.domain1.com and www.domain2.com which link to
/home/user1/public_html and /home/user2/public_html. So far this works
fine.

Every user can login via ftp, where its homedirectory is chrooted. This
works fine, too.

I want to to give every user write-permission (ftp and php). If user1
uploads a file via ftp, its owned by user1:users . Apache/PHP cannot write
to this file because apache runs as nobody.

So whats the solution to this problem? I am not very experienced with unix
permissions so please help me to understand.

thanks,

Kai