How to syntax-highlight a part of file in a different syntax?

Question

I installed bnf.vim (highlights BNF grammar files).

Suppose I have a comment in my code:

/* 

 :=  | h
 := a | b |          


        

Answer 1

for many file types vim will use the heredoc names as a clue. For example in php files I'm often creating here docs thus:

Solar 
Is good
 
html; 

$my_js = <<

works beautifully, with only problem being that indentation is problematic (the closing marker cannot have preceding spaces)