<?php

$publicKeyString = "-----BEGIN PUBLIC KEY-----

-----END PUBLIC KEY-----";
$privateKeyString = "-----BEGIN RSA PRIVATE KEY-----

-----END RSA PRIVATE KEY-----";

$publicKey = openssl_pkey_get_public($publicKeyString);
$privateKey = openssl_pkey_get_private($privateKeyString);

$data = '{"foo":"bar"}';

// encode

$crypted = null;
$result = openssl_public_encrypt($data, $crypted, $publicKey);
$cryptedBase64 = base64_encode($crypted);


// decode

$decrypted = null;
$hash = base64_decode($cryptedBase64);
openssl_private_decrypt($hash, $decrypted, $privateKey);

var_dump($decrypted);