How do I tell Vim to ignore certain "second extensions"?

Question

E.g. foo.php.tpl should be highlighted as PHP, foo.js.erb as Javascript, foo.py.tmp as Python...

Answer 1

I would add something like this in ~/.vimrc

au BufEnter,BufRead *.php.tpl set filetype=php
au BufEnter,BufRead *.js.erb set filetype=javascript
au BufEnter,BufRead *.py.tmp set filetype=python

Same as that of @leaf's answer but accounted for BufRead event also.