I think you have the terms php and sendmail screwed up a bit
Sendmail is a stand alone program for linux/unix, there is a version for windows but it´s not free.
php however have the mail()
function, all you need to do is edit these lines in php.ini
(reqires apache restart)
; For Win32 only.
SMTP = smtp.example.com
smtp_port = 25
sendmail_from = firstname.lastname@example.org
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").