paypalnvp-php is a very light and simple library for the PayPal NVP API.

paypalnvp-php is developed and maintained by Mahmoud Al-Qudsi
<mqudsi@neosmart.net> of NeoSmart Technologies <http://neosmart.net/>

paypalnvp-php is a class designed to hide the nuances and communication
with the PayPal backend from developers, without abstracting away the
NVP API. It is meant to be used in tight conjunction with the official
PayPal API reference <http://j.mp/HBkl7K>

Usage:

As mentioned above, the paypalnvp library does not attempt to abstract
the underlying concepts away from the developer. Usage is as easy as
creating an associative array of key => value pairs per the official
PayPal reference, instantiating a new PayPal(user, pass, sig) object
with the NVP credentials, then calling the desired method.

Internally, the paypalnvp library takes care of the authentication,
version, and method NVP key-value pairs. You only need to supply the
method-specific key-value pairs. The library takes care of all
case-sensitivity related issues for the key value, as well as any and
all encoding-related issues for both the key and the value.

For instance, the following is a sample usage of the
GetExpressCheckoutDetails API call:

    $paypal = new PayPal(YOUR_USER, YOUR_PASS, YOUR_SIGNATURE);
    
    $fields = array(
        "Token" => $token
    );
    
    $result = $paypal->GetExpressCheckoutDetails($fields);
    
    $invoice = $result['INVNUM'];
    $email   = $result['EMAIL'];
    $first   = $result['FIRSTNAME'];
    $last    = $result['LASTNAME'];

All the code is contained within PayPal.php and is in the neosmart
namespace.