Adding File based SDK Configurations: https://github.com/paypal/PayPal-PHP-SDK/wiki/File-based-Configurations

third.php

// 1. Autoload the SDK Package. This will include all the files and classes to your autoloader
require __DIR__  . '/PayPal-PHP-SDK/autoload.php';

// 2. Define PP_CONFIG_PATH directory
if(!defined("PP_CONFIG_PATH")) {
define("PP_CONFIG_PATH", __DIR__);
}

// 3. Lets try to create a Payment
// https://developer.paypal.com/docs/api/payments/#payment_create
$payer = new \PayPal\Api\Payer();
$payer->setPaymentMethod('paypal');

$amount = new \PayPal\Api\Amount();
$amount->setTotal('1.00');
$amount->setCurrency('USD');

$transaction = new \PayPal\Api\Transaction();
$transaction->setAmount($amount);

$redirectUrls = new \PayPal\Api\RedirectUrls();
$redirectUrls->setReturnUrl("https://example.com/your_redirect_url.html")
    ->setCancelUrl("https://example.com/your_cancel_url.html");

$payment->setIntent('sale')
    ->setPayer($payer)
    ->setTransactions(array($transaction))
    ->setRedirectUrls($redirectUrls);

// 4. Make a Create Call and print the values
try {
    $payment->create();
    echo $payment;

    echo "\n\nRedirect user to approval_url: " . $payment->getApprovalLink() . "\n";
}
catch (\PayPal\Exception\PayPalConnectionException $ex) {
    // This will print the detailed information on the exception.
    //REALLY HELPFUL FOR DEBUGGING
    echo $ex->getData();
}

I believe you have missed $payment = new \PayPal\Api\Payment(); before $payment->setIntent('sale') (line 25).