I’ve just looked at a couple of my emails, and I notice the the final attachment boundary ends with ‘–‘, while the opening boundary marker does not. In your code, you have:
--PHP-mixed-$random_hash--
Content-Type: text/plain; name=\"$filename\"
Content-Transfer-Encoding: base64
Content-Disposition: attachment
$attachment
--PHP-mixed-$random_hash--
Perhaps it should be:
--PHP-mixed-$random_hash
Content-Type: text/plain; name=\"$filename\"
Content-Transfer-Encoding: base64
Content-Disposition: attachment
$attachment
--PHP-mixed-$random_hash--
Have a look at the example here: