For the Mail functions to be available, PHP must have access to the
sendmail binary on your system during compile time.
If you use another mail program, such as qmail
or postfix, be sure to
use the appropriate sendmail wrappers that come with them. PHP will
first look for sendmail in your PATH, and then in
the following:
/usr/bin:/usr/sbin:/usr/etc:/etc:/usr/ucblib:/usr/lib.
It's highly recommended to have sendmail available from your
PATH. Also, the user that compiled PHP must have
permission to access the sendmail binary.