Working example of a simple en- and decryption using the singpolyma/openpgp-php library

messaging.php

<?php
// Encrypt a string using a public key
// $string: String to encrypt
// $key: Public key block to use when encrypting
// return: encrypted, enarmored PGP MESSAGE block
function encrypt($string, $key) {
    $key = OpenPGP_Message::parse(OpenPGP::unarmor($key, "PGP PUBLIC KEY BLOCK"));
    $data = new OpenPGP_LiteralDataPacket($string, array('format' => 'u', 'filename' => 'stuff.txt'));
    $encrypted = OpenPGP_Crypt_Symmetric::encrypt($key, new OpenPGP_Message(array($data)));
    $enc = OpenPGP::enarmor($encrypted->to_bytes(), "PGP MESSAGE");
    $enc =  wordwrap($enc, 64, "\n", 1);
    return $enc;
}

// Decrypt an encrypted, armored PGP MESSAGE block
// $encrypted: PGP MESSAGE block to decrypt
// $key: Private key block
// $pass: Password for the private key
// return: Decrypted string
function decrypt($encrypted, $key, $pass) {
    $keyEncrypted = OpenPGP_Message::parse(OpenPGP::unarmor($key, 'PGP PRIVATE KEY BLOCK'));
    $text = "";
    foreach($keyEncrypted as $p) {
        if(!($p instanceof OpenPGP_SecretKeyPacket)) continue;
        $key = OpenPGP_Crypt_Symmetric::decryptSecretKey($pass, $p);
        $msg = OpenPGP_Message::parse(OpenPGP::unarmor($encrypted, 'PGP MESSAGE'));
        $decryptor = new OpenPGP_Crypt_RSA($key);
        $decrypted = $decryptor->decrypt($msg);

        $text = $decrypted->packets[0]->data;
    }
    return $text;

}