Actually configure Postfix or Sendmail for PHP mail() is the same. Both of them run the /usr/sbin/sendmail binary. It is quite confusing if we just look at the name of the binary but this is what Postfix did.
For more information, take a look at the Postfix Manual – sendmail.
For the PHP setting, open the /etc/php5/apache2/php.ini and configure the sendmail_path. Continue reading