If you want to use mail() with headers, then you'll need to alter the signed version before embedding in the body or the headers and boundaries will end up in the message.
<?php
openssl_pkcs7_sign($basedir . 'email.txt', $basedir . 'signed.txt', 'file://' . $basedir . 'cert.pem', array('file://' . $basedir . 'key.pem', $keypass), array('To' => $smime_to, 'From' => $smime_from, 'Subject' => $smime_subject));
if (preg_match('/To: [^\r\n]+(\r|\n)+(From: [^\r\n]+(\r|\n)+)Subject: [^\r\n]+(\r|\n)+MIME-Version: [^\r\n]+(\r|\n)+(Content-Type: [^\r\n]+)(\r|\n)+/', file_get_contents($basedir . 'signed.txt'), $matches))
{
$result = mail($smime_to, $smime_subject, str_replace($matches[0], '', file_get_contents($basedir . 'signed.txt')), $mailheaders . $matches[2] . $matches[6]);
}
?>
With the signed headers removed ($matches[0]) and the headers updated with From (not part of mail()) and Content-Type ($matches[2] / $matches[6] respectively).
