Openpgp php decrypt returns null - php

Question

I am trying to implement the following openpgp-php library to encrypt my text. But when I'm trying to decrypt, it gives null!
https://github.com/singpolyma/openpgp-php
My code implementation
<?php
require_once 'vendor/autoload.php';
require_once 'lib/openpgp.php';
require_once 'lib/openpgp_crypt_rsa.php';
require_once 'lib/openpgp_crypt_symmetric.php';
$public_key_ascii = file_get_contents('mypubkey.pgp');
$key = OpenPGP_Message::parse($public_key_ascii);
$data = new OpenPGP_LiteralDataPacket('Hello world!');
$encrypted = OpenPGP_Crypt_Symmetric::encrypt($key, new OpenPGP_Message(array($data)));
echo '<pre>Encrypted object: <br/>';
var_dump($encrypted);
$decryptor = new OpenPGP_Crypt_RSA($key);
$decrypted = $decryptor->decrypt($encrypted);
echo '<br/>Decrepted: ';
var_dump($decrypted);
Now when I'm running the script I'm getting the following output:
Encrypted object:
object(OpenPGP_Message)#10 (2) {
["uri"]=>
NULL
["packets"]=>
array(1) {
[0]=>
object(OpenPGP_IntegrityProtectedDataPacket)#7 (4) {
["version"]=>
int(1)
["tag"]=>
int(18)
["size"]=>
NULL
["data"]=>
string(68) "u��0��Y
�W~�-rS}e�#�2DJ�p����l�k�FakΩ-���pk� #Y�ft�Nm�S�zt�"
}
}
}
Decrepted: NULL
Any suggestions will be appreciated. Thanks