openssl_pkcs7_decrypt
    (PHP 4 >= 4.0.6, PHP 5)
openssl_pkcs7_decrypt -- Decrypts an S/MIME encrypted message
Description
bool 
openssl_pkcs7_decrypt ( string 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.
    
The decrypted message is output to the
     file specified by outfilename
    
     
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!"; } ?>
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