PHP streaming email

send_mail.php

<?php

function send_mail_with_attachment($to, $subject, $message, $additional_headers, $file_handle, $file_name)
{
	if (!$file_handle)
		throw new InvalidArgumentException("The provided file handle is not working");

	$fout = popen('sendmail ' . escapeshellarg($to), 'w');

	if (!$fout)
		throw new Exception("Could not open sendmail");

	$boundary = md5(microtime());

	// Headers and dummy message
	fwrite($fout,
		"MIME-Version: 1.0\r\n"
		. ($additional_headers ? (trim($additional_headers, "\r\n") . "\r\n") : "")
		. "Subject: $subject\r\n"
		. "Content-Type: multipart/mixed; boundary=\"$boundary\"\r\n"
		. "\r\n"
		. "This is a mime-encoded message"
		. "\r\n\r\n");

	// Message content
	fwrite($fout, "--$boundary\r\n"
		. "Content-Type: text/plain; charset=\"UTF-8\"\r\n"
		. "Content-Transfer-Encoding: quoted-printable\r\n\r\n");

	$filter = stream_filter_append($fout, 'convert.quoted-printable-encode');

	if (is_resource($message))
		stream_copy_to_stream($message, $fout);
	else
		fwrite($fout, $message);

	stream_filter_remove($filter);

	fwrite($fout, "\r\n\r\n");

	// Attachment
	fwrite($fout, "--$boundary\r\n"
		. "Content-Type: application/octet-stream; name=\"" . addslashes($file_name) . "\"\r\n"
		. "Content-Transfer-Encoding: base64\r\n"
		. "Content-Disposition: attachment\r\n\r\n");

	$filter = stream_filter_append($fout, 'convert.base64-encode', STREAM_FILTER_WRITE, [
		"line-length" => 80,
		"line-break-chars" => "\r\n"]);

	stream_copy_to_stream($file_handle, $fout);

	stream_filter_remove($filter);

	fwrite($fout, "\r\n\r\n"
		. "--$boundary--\r\n");

	fclose($fout);
}