openssl_pkcs7_encrypt
    (PHP 4 >= 4.0.6)
openssl_pkcs7_encrypt -- Encrypt an S/MIME message
Description
bool 
openssl_pkcs7_encrypt ( string infile, string outfile, mixed recipcerts, array headers [, int flags])
     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, which is either a      lone X.509 certificate, or an array of X.509 certificates.      headers is an array of headers that      will be prepended to the data after it has been encrypted.      flags can be used to specify options that affect      the encoding process - see PKCS7      constants.      headers can be either an associative array      keyed by header name, or an indexed array, where each element contains      a single header line.     
     
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");
}
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