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but certain values such as the API Password, (PWD), are case-sensitive. The required parameters for all NVP API transactions are USER, PWD, METHOD, and VERSION. The METHOD, or type of transaction you are calling the NVP API to process, has an associated VERSION. Together the METHOD and VERSION define the exact behavior of the API operation you want performed. This will be followed by the information posted from your application, including things such as Item, Quantity, and Cost.

Tip

API operations can change between versions, so when you change a version number, I recommend retesting your application code before going live.

Figure 1-3 outlines the API operation of an NVP request, and Figure 1-4 shows the same transaction with credentials provided.

Figure 1-3. NVP request

Figure 1-4. NVP request with credentials

Putting it together

Now that we have the basic elements laid out, let’s put together a sample URL encoded NVP request via PHP, shown in Examples 1-1 and 1-2.

Example 1-1. developercredentials.php

//PayPal NVP API Test Developer Credentials//

$paypalusername = sdk-three_api1.sdk.com;

$paypalpassword = QFZCWN5HZM8VBG7Q;

$paypalsignature = A-IzJhZZjhg29XQ2qnhapuwxIDzyAZQ92FRP5dqBzVesOkzbdUONzmOU;

$paypalserver = api-3t.sandbox.paypal.com/nvp

?>

Example 1-2. simpletransactionrequestprocessor.php

// PayPal NVP API Simple Transaction Request Processor//

// Include the developercredentials.php file for relevant information

include("../path/outside/webroot/developercredentials.php");

// Build the credentials format of the Request String

$credentials= "USER=$paypaluser&PWD=$paypalpwd&SIGNATURE=$paypalsig";

// Designate the API Method we are calling to have handled

$method = api_method_to_use;

$version = method_version_to_use;

// Build Initial Request string

$request = $method."&".$version."&".$credentials;

// Walk the posted form elements to gather additional information

// to pass URLEncoded to API via the request string

foreach ($_POST as $key => $value){

$value = urlencode(stripslashes($value));

$request. = "&$key=$value";

};

//Build transaction and execute via curl

$ch = curl_init();

// Ensure communication is done via SSL and over a fully verified

// SSL key and certificate

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, TRUE);

curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, TRUE);

// Return response as a string from server

curl_setopt($ch, CURL_RETURNTRANSFER, 1);

// Post values to server via URLEncoded string

curl_setopt($ch, CURLOPT_POST, 1);

curl_setopt($ch, CURLOPT_POSTFIELDS, $request);

//Execute Request

$response = curl_exec($ch);

?>

Tip

Notice that in Example 1-2, we reference the developercredentials.php file from a path outside the webroot. As stated earlier, this will ensure that no one can access your credentials file directly from their web browser and ensures that this information stays secure. If we were satisfied with this code and wanted to go to production, we would then change this path to the location of our production credentials file.

Parsing an NVP Response

When it comes to parsing an NVP response, your application really has to accomplish only one major step: URL decoding.

URL decoding

URL decoding the response from PayPal is basically just the reverse of URL encoding the values to pass to PayPal. For example:

NAME=John+Doe&Company=Acme+Goods+%26+Services

is decoded as follows:

NAME=John Doe&COMPANY= Acme Goods & Services

As with URL encoding, each application language typically has a URL decode method built into the language. Refer to the list in Table 1-3.

Table 1-3. URL decoding methods

Application language Function Method name

ASP.NET Decode System.Web.HttpUtility.UrlDecode(buffer, Encoding.Default)

Classic ASP Decode No built-in function; several implementation examples are available on the Internet

Java Decode java.net.URLDecoder.decode

PHP Decode urldecode()

ColdFusion Decode URLDecodeurlEncodedString[, charset])

Response format

Each NVP API response is composed of an acknowledgment (or