How does PHP's `mail` work?

Question

PHP\'s mail function seems to deliver mail on a clean system, with no apparent configuration done by the administrator or webmaster (no SMTP configuration in

Answer 1

You can detect how it works as below.

First method

$ ltrace php -r "mail('tester@127.0.0.1', 'Test', 'Hello world');" 2>&1 | grep sendmail
memcpy(0x095ea168, "sendmail_from", 14)          = 0x095ea168
memcpy(0x095ea1e0, "sendmail_path", 14)          = 0x095ea1e0
popen("/usr/sbin/sendmail -t -i ", "w")          = 0x0977c7c0

From the results of the above command can be seen that the popen() function opens the process of /usr/sbin/sendmail -t -i.

$ ls -l /usr/sbin/sendmail
... /usr/sbin/sendmail -> exim4

So sendmail is the symbolic link to exim4 and hence sendmail -t -i invokes exim4 -t -i.

And in the manual page of exim4 you can read about these options -t -i:

$ man exim4 | grep ' -t -i'
-ti       This option is exactly equivalent to -t -i. It is provided for compatibility with Sendmail.

Second method

Install snoopy and run:

# grep snoopy /var/log/auth.log | tail
... php -r mail('tester@127.0.0.1', 'Test', 'Hello world');
... /usr/sbin/sendmail -t -i
... /usr/sbin/exim4 -Mc 1YxxYn-0006a7-Nw
... /usr/sbin/exim4 -t -oem -oi -f <> -E1YxxYn-0006a7-Nw
... /usr/sbin/exim4 -Mc 1YxxYn-0006aB-Oj

The results of the above command show the sequence of the commands which were performed.