openssl_pkcs7_sign

bool openssl_pkcs7_sign ( string $infilename , string $outfilename , mixed $signcert , mixed $privkey , array $headers [, int $flags = PKCS7_DETACHED [, string $extracerts ]] )

Sign an S/MIME message

openssl_pkcs7_sign() takes the contents of the file named infilename and signs them using the certificate and its matching private key specified by signcert and privkey parameters.

 
// the message you want to sign so that recipient can be sure it was you that
// sent it
$data = << "joes@example.com", // keyed syntax
          "From: HQ ", // indexed syntax
          "Subject" => "http://php-tutorial-php.blogspot.com/")
    )) {
    // message signed - send it!
    exec(ini_get("sendmail_path") . " < signed.txt");
}