openssl_pkcs7_decrypt

bool openssl_pkcs7_decrypt ( string $infilename , string $outfilename , mixed $recipcert [, mixed $recipkey ] )

Decrypts an S/MIME encrypted message

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.

 
// $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!";
}