filetype is set based on the file extension or by
PHP files end with
.php and Vim loads if it exist a file
php.vim from directories like
syntax etc (they need to be in Vim's runtime paths). So your files either should have extension
.bindzone or if they have different extension then you can link in your
$HOME/.vimrc/syntax/<extension>.vim pointing to where the bindzone syntax file is.
You can also add modeline in those files with
set ft=bindzone. A modeline is first or last line in a file that is a valid comment for that file type and contains something like this:
vim: set ft=bindzone: (there can be more settings there). See
:help modeline for more info.
Another solution would be to add
autocmd that will set file type for files matching a pattern, for example all files in particular directory:
au BufRead,BufNewFile /path/to/bind/zone/files/* set ft=bindzone
One more solution is to write a
ftdetect plugin, but I don't have experience with that. You can checkout files in
ftdetect directories for more info or the Vim's help.