The mail() function allows you to send mail
Mail Configuration Directives
Used under Windws only: DNS name or IP address of the SMTP server PHP should use for mail sent with the mail() function.
Used under Windws only: Number of the port to connect to the server specified with the SMTP setting when sending mail with mail(); defaults to 25. Only available since PHP 4.3.0.
Which "From:" mail address should be used in mail sent from PHP under Windows.
Where the sendmail program can be found, usually /usr/sbin/sendmail or /usr/lib/sendmail configure does an honest attempt of locating this one for you and set a default, but if it fails, you can set it here.
Systems not using sendmail should set this directive to the sendmail wrapper/replacement their mail system offers, if any. For example, Qmail users can normally set it to /var/qmail/bin/sendmail or /var/qmail/bin/qmail-inject.
qmail-inject does not require any option to process mail correctly.