Could you please turn the Windows-only variable 
sendmail_from into a variable that may be used in Apache
configuration files under unix?

This is for the benefit of the use of the Mail() function
in virtual hosts.  I would like to do something like this
in Apache's httpd.conf:

<VirtualHost bla.foo.org>
php3_sendmail_from webmaster@foo.org
</VirtualHost>

<VirtualHost www.bla.com>
php3_sendmail_from writeme@bla.com
</VirtualHost>

and either PHP adds a "From:" header in addtion to the
"To:" and "Subject:", or, even better, provides sendmail
with the neccessary flags for setting the SMTP envelope
From address ("-f").  This could even be configurable
via an other Apache variable, or as a compile time option
via the configure script.