Spotify Error Code 403 - This request requires user authentication - php

Question

I want to add tracks to my PRIVATE Spotify playlist by php - but i don´t get it done. I´m always getting the error message
Error Code 403 - This request requires user authentication.
I'm not the good in the spotify api. Can someone help me out? This is my code:
<?php
error_reporting(E_ALL);
$url = 'https://accounts.spotify.com/api/token';
$method = 'POST';
$credentials = "Client ID:Client Secret";
$headers = array(
"Accept: */*",
"Content-Type: application/x-www-form-urlencoded",
"Authorization: Basic " . base64_encode($credentials));
$data = 'grant_type=client_credentials';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response_1 = curl_exec($ch);
curl_close($ch);
$response_1 = json_decode($response_1, true);
var_dump($response_1);
$token = $response_1['access_token'];
echo $token."<br>";
$headers_2 = array(
"Accept: application/json",
"Content-Type: application/x-www-form-urlencoded",
('Authorization: Bearer ' . $token));
$url_2 = 'https://api.spotify.com/v1/users/example/playlists/playlist_example_id/tracks';
$postargs = '?uris=spotify%3Atrack%3A4xwB7i0OMsmYlQGGbtJllv';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url_2);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers_2);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postargs);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response_2 = curl_exec($ch);
curl_close($ch);
$response_2 = json_decode($response_2, true);
var_dump($response_2);