The ini file under *nix system is most likely
/etc/php4/apache/php.ini
if you still arent sure try running a locate
locate php.ini (If you have it installed)
The default is
sendmail_path = sendmail -t -i
for more options read the doc on sendmail (or under *nix man sendmail)... you might even giv e it an absolute path like
/usr/bin/sendmail
or something like that.
Hope this Helps!