[SOLVED] PHP verify Paypal webhook signature

Well I have come to the conclusion that the Paypal developer information is rather poor, it is scatted all over the place, on multiple different pages and sites. The examples they give on the paypal developer website HERE are not a complete picture of what is required to validate a webhook signature. Stripe developer documentation is much better formatted and concise.

Installing Paypal Checkout V2 SDK does not give you the necessary development tools to validate paypal webhook signatures i.e. you can process payments and receive webhooks but you can not validate the webhook signatures....I know stupid. Tip do not download the SDK directly as you will not include the required autoload.php file. Use composer to install Paypal Checkout V2 SDK so that you get the autoload.php file.

Once you are to a point that you can process payments and receive webhooks form paypal you need to install the another SKD called Paypal Rest API SDK. Again use composer to install the SDK so that you get a autoload.php file which you will need.

When you install Paypal Rest API SDK amazingly you will still be missing files that are required to validate the payapl webhook signatures. I can find no mention of these anywhere on the paypal developer website.

bootstrap.php & common.php

Thanks to @Grumpy I got some samples provided on github HERE

Note you will probably need to modify the samples a bit in order to get them working with your website. Tip set the logger to false and save yourself some bother if you don't have the necessary access permissions to write.

Once you have bootstrap.php & common.php files created you can write the code for your webhook endpoint page i.e. the page that paypal sends the webhook to. I have included my PHP code below for how to validate and then process the paypal webhook. Tip in the below code you need to specify the webhook ID, each webhook that you create in paypal has a unique ID. Also when you are testing you can not use webhook simulator as this will fail validation, you can make manually a payment with your sandbox account details which will trigger a webhook payment event.

Paypal sure don't make it easy, their documentation is all over the place compared to Stripe. The Paypal webhooks can sometimes take several minutes to arrive after a payment is made, very frustrating when trying to debug. Also it's a bit ridiculous that they have a webhook simulator on the paypal developer website that can not be used to validate signatures...if stripe can do it why can't paypal.

<?php





//get the webhook payload

$requestBody = file_get_contents('php://input');

//check if webhook payload has data
if($requestBody) {
//request body is set
} else {
//request body is not set
exit(); 
}




use \PayPal\Api\VerifyWebhookSignature;
use \PayPal\Api\WebhookEvent;

$apiContext = require __DIR__ . '/bootstrap.php';




//Receive HTTP headers that you received from PayPal webhook.

$headers = getallheaders();


//need header keys to be UPPERCASE

$headers = array_change_key_case($headers, CASE_UPPER);


/*

example header paypal signature content for webhook, these values are recieved as an array, we then need to use this data to verify the payload


CONTENT-LENGTH : 1376

CORRELATION-ID : 6db85170269e7

USER-AGENT : PayPal/AUHD-214.0-54377828

CONTENT-TYPE: application/json

PAYPAL-AUTH-ALGO : SHA256withRSA

PAYPAL-CERT-URL : https://api.paypal.com/v1/notifications/certs/CERT-360caa42-fca2a784-5edc0ebc

PAYPAL-AUTH-VERSION : v2

PAYPAL-TRANSMISSION-SIG : Hc2lsDedYdSjOM4/t3T/ioAVQqFPNVB/AY/EyPNlavXk5WYUfnAmt9dyEP6neAPOjFHiVkXMK+JlLODbr6dalw6i26aFQdsPXqGl38Mafuu9elPE74qgsqNferUFgHi9QFXL+UZCNYcb4mvlDePXZIIAPbB0gOuFGOdEv2uqNwTCSAa/D8aguv1/51FWb3RkytFuVwXK/XNfIEy2oJCpDs8dgtYAZeojH8qO6IAwchdSpttMods5YfNBzT7oCoxO80hncVorBtjj1zQrkoynEB9WNNN9ytepNCkT8l29fQ4Sx/WRndm/PESCqxqmRoYJoiSosxYU3bZP7QTtILDykQ==

PAYPAL-TRANSMISSION-TIME : 2020-04-05T14:40:43Z

PAYPAL-TRANSMISSION-ID : 6dec99b0-774b-11ea-b306-c3ed128f0c4b


*/


//if any of the relevant paypal signature headers are not set exit()

if(
(!array_key_exists('PAYPAL-AUTH-ALGO', $headers)) ||
(!array_key_exists('PAYPAL-TRANSMISSION-ID', $headers)) ||
(!array_key_exists('PAYPAL-CERT-URL', $headers)) ||
(!array_key_exists('PAYPAL-TRANSMISSION-SIG', $headers)) ||
(!array_key_exists('PAYPAL-TRANSMISSION-TIME', $headers)) 
)
{

exit();     
}

//specify the ID for the webhook that you have set up on the paypal developer website, each web hook that you create has a unique ID


$webhookID = "ENTER_YOUR_WEBHOOK_ID_HERE";




//start paypal webhook signature validation 

$signatureVerification = new VerifyWebhookSignature();
$signatureVerification->setAuthAlgo($headers['PAYPAL-AUTH-ALGO']);
$signatureVerification->setTransmissionId($headers['PAYPAL-TRANSMISSION-ID']);
$signatureVerification->setCertUrl($headers['PAYPAL-CERT-URL']);
$signatureVerification->setWebhookId($webhookID); 
$signatureVerification->setTransmissionSig($headers['PAYPAL-TRANSMISSION-SIG']);
$signatureVerification->setTransmissionTime($headers['PAYPAL-TRANSMISSION-TIME']);

$signatureVerification->setRequestBody($requestBody);
$request = clone $signatureVerification;

try {

$output = $signatureVerification->post($apiContext);

} catch (Exception $ex) {

//error during signature validation, capture error and exit

ResultPrinter::printError("Validate Received Webhook Event", "WebhookEvent", null, $request->toJSON(), $ex);
exit(1);

}


$sigVerificationResult = $output->getVerificationStatus();

// $sigVerificationResult is a string and will either be "SUCCESS" or "FAILURE"


//if not webhook signature failed validation exit
if($sigVerificationResult != "SUCCESS"){

exit(); 
}
else if($sigVerificationResult == "SUCCESS"){

//paypay webhook signature is valid

//proceed to process webhook payload


//decode raw request body

$requestBodyDecode = json_decode($requestBody);


//pull whatever info required from decoded request body, some examples below


$paymentSystemID = $requestBodyDecode->id;


$eventType = $requestBodyDecode->event_type;


//do something with info captured from the webhook payload


}