(PHP 4 >= 4.0.6, PHP 5)
openssl_pkcs7_decrypt — Decrypts an S/MIME encrypted message
$infilename
   , string $outfilename
   , mixed $recipcert
   [, mixed $recipkey
  ] )
   Decrypts the S/MIME encrypted message contained in the file specified by
   infilename using the certificate and its
   associated private key specified by recipcert and
   recipkey.
  
infilename
outfilename
       The decrypted message is written to the file specified by
       outfilename.
      
recipcert
recipkey
   Returns TRUE on success or FALSE on failure.
  
Example #1 openssl_pkcs7_decrypt() example
<?php
// $cert and $key are assumed to contain your personal certificate and private
// key pair, and that you are the recipient of an S/MIME message
$infilename = "encrypted.msg";  // this file holds your encrypted message
$outfilename = "decrypted.msg"; // make sure you can write to this file
if (openssl_pkcs7_decrypt($infilename, $outfilename, $cert, $key)) {
    echo "decrypted!";
} else {
    echo "failed to decrypt!";
}
?>