In [HOST=...] one should use only the 1st server name from the list of server names.
I.E. if you have
server_name example.com, example2.com;
and will use www.example.com in a browser,
use [HOST=example.com] in php.ini for both addresses.
PHP looks not at the HOST request header (as I expected), but at the SERVER_NAME parameter (which by default is the 1st from the list of names, no matter what the HOST is).