How to send email further in sendmail after stub?

Question

PHP

Hello.
I'm trying to make some layer for emails in php 7.0 in order to subject them to a little processing and most importantly - limiting outgoing emails. In php.ini, I redirected the letters to my file, where I opened the stdin stream and took the necessary data, entered it into a table, and calculated it. Now I need to send them back, and send them further in the same stream to sendmail. I googled this for a very long time and still haven't found a solution.
PS I do everything locally in docker, I don’t have a mail server, I have Postfix (in fact, you can send it to postfix, but here I can’t even understand how), maybe later I will configure all this on the server.
PSS I don't need similar solutions to the whole problem, I need to do just that.
Here is a cut off part of the code, without logic and other things, the most necessary for understanding the issue.

#!/usr/bin/php
<?php
$stream = '';
$fp = fopen('php://stdin','r');

while($t=fread($fp,2048))
{
    if( $t===chr(0) )
        break;
    $stream .= $t;
}
//$count_mails - количество писем с определенного домена. 
if($count_mails > 50){
    exit(0);
}
//вот здесь должно быть что-то что отправит письма дальше после всех изысканий.

Answer 1

Hello, the solution was the line:
That is, we sent the stream further to sendmail, where it actually went.
I caught the letters using a fake mail server, as soon as the idea came up to install it, the solution was found quickly by poking and testing.

Answer 2

If postfix is ​​configured in the docker, then from php you can send the mail() function php.net/manual/ru/function.mail.php.
You can use SMTP, for example Yandex https://github.com/PHPMailer/PHPMailer