PHP OAuth 2 Implementation for PayPal Access

common.php

<?php
define('KEY', 'YOUR APPLICATION ID');
define('SECRET', 'YOUR APPLICATION SECRET');

define('CALLBACK_URL', 'YOUR CALLBACK PATH - TO COMPLETE.PHP');
define('AUTHORIZATION_ENDPOINT', 'https://identity.x.com/xidentity/resources/authorize');
define('ACCESS_TOKEN_ENDPOINT', 'https://identity.x.com/xidentity/oauthtokenservice');
define('PROFILE_ENDPOINT', 'https://identity.x.com/xidentity/resources/profile/me');

/***************************************************************************
 * Function: Run CURL
 * Description: Executes a CURL request
 * Parameters: url (string) - URL to make request to
 *             method (string) - HTTP transfer method
 *             headers - HTTP transfer headers
 *             postvals - post values
 **************************************************************************/
function run_curl($url, $method = 'GET', $postvals = null){
    $ch = curl_init($url);
    
    //GET request: send headers and return data transfer
    if ($method == 'GET'){
        $options = array(
            CURLOPT_URL => $url,
            CURLOPT_RETURNTRANSFER => 1,
            CURLOPT_SSLVERSION => 3
        );
        curl_setopt_array($ch, $options);
    //POST / PUT request: send post object and return data transfer
    } else {
        $options = array(
            CURLOPT_URL => $url,
            CURLOPT_POST => 1,
            CURLOPT_VERBOSE => 1,
            CURLOPT_POSTFIELDS => $postvals,
            CURLOPT_RETURNTRANSFER => 1,
            CURLOPT_SSLVERSION => 3
        );
        curl_setopt_array($ch, $options);
    }
    
    $response = curl_exec($ch);
    curl_close($ch);
    
    return $response;
}
?>

complete.php

<?php
require_once "common.php";

//capture code from auth
$code = $_GET["code"];

//construct POST object for access token fetch request
$postvals = sprintf("client_id=%s&client_secret=%s&grant_type=authorization_code&code=%s&redirect_uri=%s", KEY, SECRET, $code, urlencode(CALLBACK_URL));

//get JSON access token object (with refresh_token parameter)
$token = json_decode(run_curl(ACCESS_TOKEN_ENDPOINT, 'POST', $postvals));

//construct URI to fetch profile information for current user
$profile_url = sprintf("%s?oauth_token=%s", PROFILE_ENDPOINT, $token->access_token);

//fetch profile of current user
$profile = run_curl($profile_url);

var_dump($profile);
?>

index.php

<?php
require_once "common.php";

$auth_url = sprintf("%s?scope=%s&response_type=code&redirect_uri=%s&client_id=%s",
                    AUTHORIZATION_ENDPOINT,
                    urlencode("https://identity.x.com/xidentity/resources/profile/me"),
                    urlencode(CALLBACK_URL),
                    KEY);

//forward user to PayPal auth page
header("Location: $auth_url");
?>