如何将以下Node.js加密缓冲区转换为PHP？

I am having some hard time figuring out how to convert the following code to PHP;

const envelope = JSON.stringify({ v: value, a: Cryptiles.randomString(2) });
const cipher = Crypto.createCipher('aes256', key);

let enc = cipher.update(envelope, 'utf8', 'binary');
enc += cipher.final('binary');

return (new Buffer(enc, 'binary')).toString('base64').replace(/\+/g, '-').replace(/\//g, ':').replace(/\=/g, '');

Any thoughts?