bool openssl_private_encrypt ( string $data , string &$crypted , mixed $key [, int $padding = OPENSSL_PKCS1_PADDING ] )
openssl_private_encrypt() encrypts data with private key and stores the result into crypted. Encrypted data can be decrypted via openssl_public_decrypt(). This function can be used e.g. to sign data (or its hash) to prove that it is not written by someone else.
Returns TRUE on success or FALSE on failure.