(PHP 4 >= 4.0.6, PHP 5)
openssl_pkcs7_encrypt — Encrypt an S/MIME message
$infile
   , string $outfile
   , mixed $recipcerts
   , array $headers
   [, int $flags = 0
   [, int $cipherid = OPENSSL_CIPHER_RC2_40
  ]] )
   openssl_pkcs7_encrypt() takes the contents of the
   file named infile and encrypts them using an RC2
   40-bit cipher so that they can only be read by the intended recipients
   specified by recipcerts.
  
infile
outfile
recipcertsEither a lone X.509 certificate, or an array of X.509 certificates.
headers
       headers is an array of headers that
       will be prepended to the data after it has been encrypted.
      
       headers can be either an associative array
       keyed by header name, or an indexed array, where each element contains
       a single header line.
      
flags
       flags can be used to specify options that affect
       the encoding process - see PKCS7
       constants.
      
cipheridOne of cipher constants.
   Returns TRUE on success or FALSE on failure.
  
Example #1 openssl_pkcs7_encrypt() example
<?php
// the message you want to encrypt and send to your secret agent
// in the field, known as nighthawk.  You have his certificate
// in the file nighthawk.pem
$data = <<<EOD
Nighthawk,
Top secret, for your eyes only!
The enemy is closing in! Meet me at the cafe at 8.30am
to collect your forged passport!
HQ
EOD;
// load key
$key = file_get_contents("nighthawk.pem");
// save message to file
$fp = fopen("msg.txt", "w");
fwrite($fp, $data);
fclose($fp);
// encrypt it
if (openssl_pkcs7_encrypt("msg.txt", "enc.txt", $key,
    array("To" => "nighthawk@example.com", // keyed syntax
          "From: HQ <hq@example.com>", // indexed syntax
          "Subject" => "Eyes only"))) {
    // message encrypted - send it!
    exec(ini_get("sendmail_path") . " < enc.txt");
}
?>