PayPal REST API with PHP results ERROR - php

Question

Im using PayPal REST API with PHP, I have created a rest app in sandbox and everything as in documentation.
but it returns error
Got Http response code 401 when accessing https://api.sandbox.paypal.com/v1/oauth2/token.
not sure where Im going wrong on this, tried many other sample codes using rest api but same error everywhere.
can some one help me on this?
code is here
define('CLIENT_ID', 'MY CLIENT ID'); //your PayPal client ID
define('CLIENT_SECRET', 'MY SECRET'); //PayPal Secret
define('RETURN_URL', 'http://domain.com/order_process.php'); //return URL where PayPal redirects user
define('CANCEL_URL', 'http://domain.com/payment_cancel.html'); //cancel URL
define('PP_CURRENCY', 'USD'); //Currency code
define('PP_CONFIG_PATH', ''); //PayPal config path (sdk_config.ini)
include_once "vendor/autoload.php"; //include PayPal SDK
include_once "functions.inc.php"; //our PayPal functions
$item_name = 'Test Product'; //get item code
$item_code = 'sku123'; //get item code
$item_price = '10'; //get item price
$item_qty = '1'; //get quantity
/*
Note: DO NOT rely on item_price you get from products page, in production mode get only "item code"
from the products page and then fetch its actual price from Database.
Example :
$results = $mysqli->query("SELECT item_name, item_price FROM products WHERE item_code= '$item_code'");
while($row = $results->fetch_object()) {
$item_name = $row->item_name;
$item_price = item_price ;
}
*/
//set array of items you are selling, single or multiple
$items = array(
array('name'=> $item_name, 'quantity'=> $item_qty, 'price'=> $item_price, 'sku'=> $item_code, 'currency'=>PP_CURRENCY)
);
//calculate total amount of all quantity.
$total_amount = ($item_qty * $item_price);
try{ // try a payment request
//if payment method is paypal
$result = create_paypal_payment($total_amount, PP_CURRENCY, '', $items, RETURN_URL, CANCEL_URL);
//if payment method was PayPal, we need to redirect user to PayPal approval URL
if($result->state == "created" && $result->payer->payment_method == "paypal"){
$_SESSION["payment_id"] = $result->id; //set payment id for later use, we need this to execute payment
header("location: ". $result->links[1]->href); //after success redirect user to approval URL
exit();
}
}catch(PPConnectionException $ex) {
echo parseApiError($ex->getData());
} catch (Exception $ex) {
echo $ex->getMessage();
}