What does sendmail send through /etc/mail/aliases?

Question

send mail

Tell me, according to the description here https://www.freebsd.org/doc/ru/books/handbook/send...
And the last example shows sending mail to the program, in this case the mail message is forwarded through the UNIX® channel to the standard input / usr/local/bin/procmail.
Those. as I understand it, data is being sent and you can accept it with the same php
/home/public_html/mail.php
If so, then the question arises, what should I accept?