Set payee billing plan

[ClientMBC]

I'm trying to use Paypal API SDK in PHP.

For a normal transaction, I can set a `Payee` like this :

$payee = new \PayPal\Api\Payee();
$payee->setEmail("client-test_at_xxx.com");

And then :

$transaction = new \PayPal\Api\Transaction();
$transaction->setAmount($amount)
    ->setPayee($payee);

How can do the same with a billing plan?