Authenticating with OAuth 2.0 Client Credentials Grant

<?php
require 'vendor/autoload.php';

use GuzzleHttp\Client;
use GuzzleHttp\Exception\GuzzleException;

class OAuthClientCredentials {
    private $client_id;
    private $client_secret;
    private $token_url;
    private $scope;
    private $access_token = null;
    private $expires_at = 0;

    public function __construct($clientId, $clientSecret, $tokenUrl, $scope = '') {
        $this->client_id = $clientId;
        $this->client_secret = $clientSecret;
        $this->token_url = $tokenUrl;
        $this->scope = $scope;
    }

    private function getAccessToken() {
        if ($this->access_token && $this->expires_at > time()) {
            return $this->access_token;
        }

        $client = new Client();
        try {
            $response = $client->post($this->token_url, [
                'form_params' => [
                    'grant_type' => 'client_credentials',
                    'client_id' => $this->client_id,
                    'client_secret' => $this->client_secret,
                    'scope' => $this->scope,
                ],
                'headers' => [
                    'Accept' => 'application/json',
                ],
            ]);

            $data = json_decode($response->getBody()->getContents(), true);
            if (isset($data['access_token'])) {
                $this->access_token = $data['access_token'];
                // Set expiry time a bit before actual expiry for safety
                $this->expires_at = time() + ($data['expires_in'] ?? 3600) - 60;
                return $this->access_token;
            }
        } catch (GuzzleException $e) {
            error_log("OAuth Token Error: " . $e->getMessage());
            // Handle error, e.g., throw exception or return null
        }
        return null;
    }

    public function makeAuthenticatedRequest($apiUrl, $method = 'GET', $options = []) {
        $token = $this->getAccessToken();
        if (!$token) {
            throw new Exception("Could not retrieve access token.");
        }

        $client = new Client();
        $headers = array_merge(["Authorization" => "Bearer " . $token], $options['headers'] ?? []);
        unset($options['headers']); // Avoid duplicating headers if passed in options

        try {
            $response = $client->request($method, $apiUrl, array_merge($options, ['headers' => $headers]));
            return json_decode($response->getBody()->getContents(), true);
        } catch (GuzzleException $e) {
            error_log("API Request Error: " . $e->getMessage());
            throw new Exception("API request failed: " . $e->getMessage());
        }
    }
}

// Example Usage:
// $clientId = 'YOUR_CLIENT_ID';
// $clientSecret = 'YOUR_CLIENT_SECRET';
// $tokenUrl = 'https://example.com/oauth/token';
// $apiUrl = 'https://api.example.com/data';

// try {
//     $oauthClient = new OAuthClientCredentials($clientId, $clientSecret, $tokenUrl);
//     $data = $oauthClient->makeAuthenticatedRequest($apiUrl);
//     print_r($data);
// } catch (Exception $e) {
//     echo "Error: " . $e->getMessage();
// }