PayPal Express Error: Unexpected token S in JSON at position 0 - php

Question

I am integrating PayPal express checkout (server-side) in my PHP website. While I am integrating the PayPal express popup is not opening and it is giving me "Unexpected token S in JSON at position 0" in the console. Here is the screenshot https://prnt.sc/10f10qi
Here is HTML code:
<script src="https://www.paypal.com/sdk/js?client-id=<client-id>"></script>
<script>
paypal.Buttons({
createOrder: function() {
return fetch('paypal/create-payment.php', {
method: 'post',
headers: {
'content-type': 'application/json'
}
}).then(function(res) {
return res.json();
}).then(function(data) {
return data.id;
});
},
onAuthorize: function(data, actions) {
return actions.payment.execute().then(function() {
alert(data.orderID);
});
}
}).render('#paypal-button');
</script>
<div id="paypal-button"></div>
Here is the server side code for create-payment.php
<?php
namespace Sample\CaptureIntentExamples;
require __DIR__ . '/vendor/autoload.php';
//1. Import the PayPal SDK client that was created in `Set up Server-Side SDK`.
use Sample\PayPalClient;
use PayPalCheckoutSdk\Orders\OrdersCreateRequest;
require 'paypal-client.php';
class CreateOrder
{
// 2. Set up your server to receive a call from the client
/**
*This is the sample function to create an order. It uses the
*JSON body returned by buildRequestBody() to create an order.
*/
public static function createOrder($debug=false)
{
$request = new OrdersCreateRequest();
$request->prefer('return=representation');
$request->body = self::buildRequestBody();
// 3. Call PayPal to set up a transaction
$client = PayPalClient::client();
$response = $client->execute($request);
if ($debug)
{
print "Status Code: {$response->statusCode}\n";
print "Status: {$response->result->status}\n";
print "Order ID: {$response->result->id}\n";
print "Intent: {$response->result->intent}\n";
print "Links:\n";
foreach($response->result->links as $link)
{
print "\t{$link->rel}: {$link->href}\tCall Type: {$link->method}\n";
}
// To print the whole response body, uncomment the following line
// echo json_encode($response->result, JSON_PRETTY_PRINT);
}
// 4. Return a successful response to the client.
return $response;
}
/**
* Setting up the JSON request body for creating the order with minimum request body. The intent in the
* request body should be "AUTHORIZE" for authorize intent flow.
*
*/
private static function buildRequestBody()
{
return array(
'intent' => 'CAPTURE',
'application_context' =>
array(
'return_url' => 'https://example.com/return',
'cancel_url' => 'https://example.com/cancel'
),
'purchase_units' =>
array(
0 =>
array(
'amount' =>
array(
'currency_code' => 'USD',
'value' => '220.00'
)
)
)
);
}
}
/**
*This is the driver function that invokes the createOrder function to create
*a sample order.
*/
if (!count(debug_backtrace()))
{
CreateOrder::createOrder(true);
}
?>
Please let me know what I am doing wrong? Thanks!