PHP must support openssl functions and you probably want to set the location of 
the root CA's. This may be a file or directory. eg.:

$config['smime_root_cert_location']='/etc/ssl/certs';

$config['smime_sign_extra_certs']='/file/to/extra/sign/cert';

See http://php.net/manual/en/function.openssl-pkcs7-sign.php
