How to Access PayPal Express Checkout API in PHP

Configure Your PayPal API Credentials

• Obtain your API credentials (API username, password, and signature). These can be accessed through the PayPal Developer portal under your account's API credentials section.
 
• Make sure your account is set up to use PayPal Express Checkout, either in sandbox or production mode, depending on your environment.

Set Up Your PHP Environment

• Ensure your server environment has PHP and cURL enabled, as cURL will be used to handle API requests.
• Install any necessary dependencies using a package manager like Composer if needed.

Write the API Request Code

• Create a PHP script for generating and handling API requests to the PayPal Express Checkout.
• Construct the API endpoint URL. For sandbox: `https://api-3t.sandbox.paypal.com/nvp`, for live: `https://api-3t.paypal.com/nvp`.

$api_endpoint = "https://api-3t.sandbox.paypal.com/nvp";

Build the Request Parameters

• Create an associative array for storing your API request parameters such as `METHOD`, `VERSION`, `USER`, `PWD`, `SIGNATURE`, along with any other necessary fields for the 'SetExpressCheckout' API method.
• Make sure to specify the return and cancel URLs, which direct the customer after they login to PayPal and accept or cancel the payment.

$request_params = array(
    'METHOD' => 'SetExpressCheckout',
    'USER' => 'your_api_username',
    'PWD' => 'your_api_password',
    'SIGNATURE' => 'your_api_signature',
    'VERSION' => '93',
    'PAYMENTREQUEST_0_AMT' => '20.00',
    'PAYMENTREQUEST_0_CURRENCYCODE' => 'USD',
    'RETURNURL' => 'http://yourwebsite.com/success.php',
    'CANCELURL' => 'http://yourwebsite.com/cancel.php'
);

Make the API Call

• Convert your request parameters into a format suitable for HTTP POST requests.
• Use cURL to initialize and execute the request to the PayPal's server.
• Handle the response provided by PayPal and check the API result.

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $api_endpoint);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);

// Convert request array to NVP string
$nvp_string = http_build_query($request_params);
curl_setopt($ch, CURLOPT_POSTFIELDS, $nvp_string);
$response = curl_exec($ch);

if (!$response) {
    exit("CURL Error : " . curl_error($ch));
}

curl_close($ch);

// Processing the response
parse_str($response, $parsed_response);
if ($parsed_response["ACK"] == "Success") {
    echo "Express Checkout Token: " . $parsed_response["TOKEN"];
} else {
    echo "Error: " . $parsed_response["L_LONGMESSAGE0"];
}

Redirect to PayPal

• Once you retrieve a token from the 'SetExpressCheckout' response, redirect the user to the PayPal for approval.
• Construct the redirect URL using the token received in the previous step.

header("Location: https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=" . $parsed_response["TOKEN"]);
exit();

Handle the Return and Complete the Payment

• On the return URL, retrieve the token and payer ID in order to complete the payment with a 'DoExpressCheckoutPayment' API call.
• Check the transaction for any errors and handle the logic on success or failure.

// After paypal returns to your RETURNURL, you will have token and payer ID
$token = $_GET['token'];
$payer_id = $_GET['PayerID'];

$payment_params = array(
    'METHOD' => 'DoExpressCheckoutPayment',
    'USER' => 'your_api_username',
    'PWD' => 'your_api_password',
    'SIGNATURE' => 'your_api_signature',
    'VERSION' => '93',
    'TOKEN' => $token,
    'PAYERID' => $payer_id,
    'PAYMENTREQUEST_0_AMT' => '20.00',
    'PAYMENTREQUEST_0_CURRENCYCODE' => 'USD'
);

// Similar cURL request as before to complete payment