I can not resist the advice to use ssh (scp / sftp) rather then ftp as I think it is more secure.
At any rate, vim will open files or save files over ftp (and scp).
Notice the double
See this tutorial for details.
At any rate, to save a file you are working on locally over ftp, user Nwrite
You can map that in vim with
cmap <Leader>r :Nread ftp://user@server/path/to/file.php
cmap <Leader>w :Nwrite ftp://user@server/path/to/file.php
Leader is a variable, by default it is /, but you can define it as another key
:let mapleader = "+"
For details See: