Actually a theoretical question.
say that my site consists of a form that uses PHP's email features to send email. I have 500 users who click the submit button at the same time. Now from PHP you need to send 500 letters in 500 different sessions.
will it be done at the same time? how many threads are involved? will each send a block to others and do it one by one?
There are two things you need to think about.
-, -. Apache, , . prefork, . . , Apache . - . forking threading . , Apache .
, , - PHP. PHP , . , Apache mod_php, - . ( PHP .)
tl; dr: Apache + PHP = Apache. 20-30 Apache, 20-30 . .
Linux sendmail. . MTA (), , .
sendmail