His "$headers .=" is totally correct.
"$headers .=" means "add to string $headers"
That's ok and will work.
It depends, what kind of "mailer" you are using. sendmail requires "\r\n" at the end of the header-Line.
on my linux-box with sendmail this works:
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/plain; charset=iso-8859-1\r\n";
$headers .= "From: $sentby <$sentby>\r\n";
@mail("$sendto", $msubject , $sendmsg, $headers);
Don't ask me "win32"-stuff 😃 a webserver with php on windows is always a pain in the ... :p I'am NT-Admin... but for php-coding i prefer a linux-box