public function requestSetExpressCheckout($amount, $returnUrl, $cancelUrl, array $optionalParameters = array())
    {
        return $this->sendApiRequest(array_merge($optionalParameters, array(
            'METHOD' => 'SetExpressCheckout',
            'PAYMENTREQUEST_0_AMT' => $this->convertAmountToPaypalFormat($amount),
            'RETURNURL' => $returnUrl,
            'CANCELURL' => $cancelUrl,
        )));
    }
public function requestSetExpressCheckout($amount, $returnUrl, $cancelUrl, array $optionalParameters = array())
    {
        return $this->sendApiRequest(array_merge($optionalParameters, array(
            'METHOD' => 'SetExpressCheckout',
            'PAYMENTREQUEST_0_AMT' => $this->convertAmountToPaypalFormat($amount),
            'RETURNURL' => $returnUrl,
            'CANCELURL' => $cancelUrl,
        )));
    }
public function requestSetExpressCheckout ( $ amount , $ returnUrl , $ cancelUrl , array $ optionalParameters = array ( ) ) { return $ this -> sendApiRequest ( array_merge ( $ optionalParameters , array ( 'METHOD' => 'SetExpressCheckout' , 'PAYMENTREQUEST_0_AMT' => $ this -> convertAmountToPaypalFormat ( $ amount ) , 'RETURNURL' => $ returnUrl , 'CANCELURL' => $ cancelUrl , ) ) ) ; }
Initiates an ExpressCheckout payment process.

Optional parameters can be found here:
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_SetExpressCheckout

@param float  $amount
@param string $returnUrl
@param string $cancelUrl
@param array  $optionalParameters

@return Response
Initiates an ExpressCheckout payment process .