You need to have some sort of mail handling package installed. It might be qmail, it might be exmh, it might be postfix, or it might be genuine sendmail. In any case, it is conventional for the package to include a sendmail alias so that it appears to be sendmail from the point of view of other mail-handling software.
The usual places to look are /usr/sbin/sendmail and /usr/lib/sendmail.
If this is your own machine, you should be able to locate the sendmail program and adjust php.ini accordingly. If it's not your own machine, contact your system administrator.