PayPal\Api\Payment::addBillingAgreementToken PHP Method

    public function addBillingAgreementToken($billingAgreementToken)
    {
        if (!$this->getBillingAgreementTokens()) {
            return $this->setBillingAgreementTokens(array($billingAgreementToken));
        } else {
            return $this->setBillingAgreementTokens(array_merge($this->getBillingAgreementTokens(), array($billingAgreementToken)));
        }
    }