$store_me = Sodiumcrypto_box_seal(
    $plaintext,
    $recipient_public_key
);

$visible = Sodiumcrypto_box_seal_open(
    $store_me,
    $recipient_keypair
);

/**
 * A human-usable variant of openssl_seal()
 *
 * @param string $plaintext Your message
 * @param string $publickey_string PEM-encoded RSA public key
 * @param boolean $encode Hex-encode the output?
 *
 * @return string
 */
function easy_seal($plaintext, $publickey_string, $encode = false)
{
    $pubkey = openssl_get_publickey($publickey_string);
    if ($pubkey === false) {
        throw new Exception('Could not load public key');
    }
    $sealed = '';
    $ekeys = [];
    $result = openssl_seal($plaintext, $sealed, $ekeys, [$pubkey], 'aes-256-gcm');
    if ($result === false) {
        throw new Exception('openssl_seal failed!');
    }
    if ($encode) {
        $sealed = bin2hex($sealed);
        foreach ($ekeys as $i => $key) {
            $ekeys[$i] = bin2hex($key);
        }
    }
    return json_encode([$sealed, $ekeys]);
}

/**
 * Inverse operation of easy_seal()
 *
 * @param string $ciphertext (the output of easy_seal())
 * @param string $privatekey_string PEM-encoded RSA private key
 * @param boolean $encoded Do we need to decode from hex?
 *
 * @return string
 */
function easy_unseal($ciphertext, $privatekey_string, $encoded = false)
{
    list($sealed, $ekeys) = json_decode($ciphertext, true);
    if ($encoded) {
        $sealed = hex2bin($sealed);
        foreach ($ekeys as $i => $key) {
            $ekeys[$i] = hex2bin($key);
        }
    }
    $open_data = '';
    $privkey = openssl_get_privatekey($privatekey_string);
    if ($privkey === false) {
        throw new Exception('Could not load public key');
    }

    $result = openssl_open($sealed, $open_data, $ekeys, $privkey, 'aes-256-gcm');
    if ($result === false) {
        throw new Exception('openssl_open failed!');
    }
    return $open_data;
}

$public_key = file_get_contents('/path/to/publickey.pem');
$plaintext = 'Something something dark side';
$store_me = easy_seal($plaintext, $public_key);

// Elsewhere:
$secret_key = file_get_contents('/path/to/secretkey.pem');
$visible = easy_unseal($store_me, $secret_key);