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).

this HOST and PATH sections doesn't seems to work in php.ini under apache 2.4 with php-fpm 7.1.16 for directive php_value or php_admin_value open_basedir or include_path.
phpinfo() says "none" in master and local values.

If you have a system which uses separated PHP inis for loading extensions (Debian, Fedora and most other distributions do so), [PATH=] or [HOST=] will prevent loading extensions defined in those ini files (extension= and zend_extension=).
I had some headache, when I added a hosts.ini to /etc/php5/cgi/conf.d (Debians default), where I wanted to define some defaults for a host name. The manual says extension= and zend_extension= aren't allowed in [HOST=] and [PATH=] (see above). I figured out that you can easily fix that with adding [PHP] after your definitions.
For example hosts.ini:
[HOST=dev.example.com]
display_errors = on
[PHP]
this will change the section back to [PHP] where extension= and zend_extension= is allowed.

Just developed the probably first php.ini hack to add commands which will only be parsed before PHP 5.3 when using mod_php.
<?php
# Disable eAccelerator by default
eaccelerator.enable = 0
# Now, to prevent incompatibilities with Zend Optimizer+, we
# want to enable eAccelerator only in PHP 5.2 or lower.
[HOST=*]
eaccelerator.enable = 1
?>

What is happening here? In PHP 5.3, php.ini sections to set up PHP on a per host basis have been introduced:
http://www.php.net/manual/en/ini.sections.php
These sections only work in CGI/FastCGI mode, however there still seems to be a difference in how the php.ini file is parsed in PHP 5.2 and PHP 5.3 when using mod_php: PHP 5.3 ignores everything below the line [HOST=*], whereas PHP 5.2 does not.
In the above example, PHP 5.3 only reads "eaccelerator.enable = 0" and then stops at the invalid command [HOST=*]. However, PHP 5.2 seems to ignore the invalid command and parses the whole configuration file, ending up with "eaccelerator.enable = 1".

Note that you cannot usefully set syslog.* in [PATH=] (and probably [HOST=]) sections, since the syslogger is initialized only once per php-fpm worker process, and not per request.

Sorry for the note. I've read some bad tutorials.
The correct syntax is : directive = 'value'
not a thing like : php_value[directive] = "somevalue"

On Windows IIS 7 server I noticed that [PATH=] was not listened to. [HOST=] however worked.

Adding PATH= applies only to the named path and does not include sub-directories, you have to add a PATH= value for each sub-directory.
Also some settings have no effect even though phpinfo recognises and displays the per-folder value as the 'local' value, and the default as the 'master' value.
'fastcgi.impersonate' always uses the master setting.