Api.php
protected function doFunction_setExpressCheckout(array $fields)
    {
        $request = new FormRequest;
        $request->setFields($fields);

        if (!isset($fields['return_url'])) {
            if (false == $this->options['return_url']) {
                throw new RuntimeException('The return_url must be set either to FormRequest or to options.');
            }

            $request->setField('return_url', $this->options['return_url']);
        }

        if (!isset($fields['cancel_url'])) {
            if (false == $this->options['cancel_url']) {
                throw new RuntimeException('The cancel_url must be set either to FormRequest or to options.');
            }

            $request->setField('cancel_url', $this->options['cancel_url']);
        }

        $request->setField('function', 'SetExpressCheckout');

        $this->addVersionField($request);
        $this->addAuthorizeFields($request);

        return $this->doRequest($request);
    }
protected function doFunction_setExpressCheckout(array $fields)
    {
        $request = new FormRequest;
        $request->setFields($fields);

        if (!isset($fields['return_url'])) {
            if (false == $this->options['return_url']) {
                throw new RuntimeException('The return_url must be set either to FormRequest or to options.');
            }

            $request->setField('return_url', $this->options['return_url']);
        }

        if (!isset($fields['cancel_url'])) {
            if (false == $this->options['cancel_url']) {
                throw new RuntimeException('The cancel_url must be set either to FormRequest or to options.');
            }

            $request->setField('cancel_url', $this->options['cancel_url']);
        }

        $request->setField('function', 'SetExpressCheckout');

        $this->addVersionField($request);
        $this->addAuthorizeFields($request);

        return $this->doRequest($request);
    }
Require:

@param array $fields

@return array