Simple PHP example of using Github's OAuth 2 API. (Please see comments for PHP 8.1 later and latest API version.)

github-oauth2-client.php

<?php
define('OAUTH2_CLIENT_ID', '');
define('OAUTH2_CLIENT_SECRET', '');

$authorizeURL = 'https://github.com/login/oauth/authorize';
$tokenURL = 'https://github.com/login/oauth/access_token';
$apiURLBase = 'https://api.github.com/';

session_start();

// Start the login process by sending the user to Github's authorization page
if(get('action') == 'login') {
  // Generate a random hash and store in the session for security
  $_SESSION['state'] = hash('sha256', microtime(TRUE).rand().$_SERVER['REMOTE_ADDR']);
  unset($_SESSION['access_token']);

  $params = array(
    'client_id' => OAUTH2_CLIENT_ID,
    'redirect_uri' => 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'],
    'scope' => 'user',
    'state' => $_SESSION['state']
  );

  // Redirect the user to Github's authorization page
  header('Location: ' . $authorizeURL . '?' . http_build_query($params));
  die();
}

// When Github redirects the user back here, there will be a "code" and "state" parameter in the query string
if(get('code')) {
  // Verify the state matches our stored state
  if(!get('state') || $_SESSION['state'] != get('state')) {
    header('Location: ' . $_SERVER['PHP_SELF']);
    die();
  }

  // Exchange the auth code for a token
  $token = apiRequest($tokenURL, array(
    'client_id' => OAUTH2_CLIENT_ID,
    'client_secret' => OAUTH2_CLIENT_SECRET,
    'redirect_uri' => 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'],
    'state' => $_SESSION['state'],
    'code' => get('code')
  ));
  $_SESSION['access_token'] = $token->access_token;

  header('Location: ' . $_SERVER['PHP_SELF']);
}

if(session('access_token')) {
  $user = apiRequest($apiURLBase . 'user');

  echo '<h3>Logged In</h3>';
  echo '<h4>' . $user->name . '</h4>';
  echo '<pre>';
  print_r($user);
  echo '</pre>';

} else {
  echo '<h3>Not logged in</h3>';
  echo '<p><a href="?action=login">Log In</a></p>';
}


function apiRequest($url, $post=FALSE, $headers=array()) {
  $ch = curl_init($url);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);

  if($post)
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post));

  $headers[] = 'Accept: application/json';

  if(session('access_token'))
    $headers[] = 'Authorization: Bearer ' . session('access_token');

  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

  $response = curl_exec($ch);
  return json_decode($response);
}

function get($key, $default=NULL) {
  return array_key_exists($key, $_GET) ? $_GET[$key] : $default;
}

function session($key, $default=NULL) {
  return array_key_exists($key, $_SESSION) ? $_SESSION[$key] : $default;
}

Thank you so much for sharing this quick piece of code. I have tested, but it didn't work because it lacked of
"https" so I updated the piece of code below...:

<?php

// Code taken from https://gist.github.com/asika32764/b204ff4799d577fd4eef
// Code changes by @edutrul "added https" in all responses
// Makes sure when creating the app you deploy to a hosting/cloud
// If you want to do it quickly then use heroku.
// When adding the app in github make sure you just add the domain name like https://example.com
// Notice the "https" being added in the callback, website and other fields in github page.

define('OAUTH2_CLIENT_ID', '<YOUR CLIENT ID>');
define('OAUTH2_CLIENT_SECRET', '<YOUR CLIENT SECRET GENERATED ONCE in GITHUB APP - make sure to take notes>');

# URL of github api
$authorizeURL = 'https://github.com/login/oauth/authorize';
$tokenURL = 'https://github.com/login/oauth/access_token';
$apiURLBase = 'https://api.github.com/';

# start sessions
session_start();

print_r($_SESSION);

// Start the login process by sending the user to Github's authorization page
if (get('action') == 'login') {
  // Generate a random hash and store in the session for security
  $_SESSION['state'] = hash('sha256', microtime(TRUE).rand().$_SERVER['REMOTE_ADDR']);
  unset($_SESSION['access_token']);

  $params = array(
    'client_id' => OAUTH2_CLIENT_ID,
    'redirect_uri' => 'https://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'],
    'scope' => 'user',
    'state' => $_SESSION['state']
  );

  // Redirect the user to Github's authorization page
  header('Location: ' . $authorizeURL . '?' . http_build_query($params));
  die();
}

// to kill all Sessions and reset code base
if (get('action') == 'exit') {
  unset($_SESSION['state']);
  unset($_SESSION['access_token']);
  session_destroy();
  exit();
}

// When Github redirects the user back here, there will be a "code" and "state" parameter in the query string
if (get('code')) {
  // Verify the state matches our stored state
  if(!get('state') || $_SESSION['state'] != get('state')) {
    header('Location: ' . $_SERVER['PHP_SELF']);
    die();
  }

  // Exchange the auth code for a token
  $token = apiRequest($tokenURL, array(
    'client_id' => OAUTH2_CLIENT_ID,
    'client_secret' => OAUTH2_CLIENT_SECRET,
    'redirect_uri' => 'https://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'],
    'state' => $_SESSION['state'],
    'code' => get('code')
  ));
  $_SESSION['access_token'] = $token->access_token;

  header('Location: ' . $_SERVER['PHP_SELF']);
}

# if successful show results
if (session('access_token')) {
  $user = apiRequest($apiURLBase.'user');

  echo '<h3>Logged In</h3>';
  echo '<h4>' . $user->login . '</h4>';
  echo '<pre>';
  print_r($user);
  echo '</pre>';


# print out full list of urls of github
  print '<br /><br />';
  print '<h3>Full List of Urls on Github</h3>';
  $full = apiRequest($apiURLBase);
  foreach ($full as $key=>$value)
  {
    print $key .'=>'. $value.'<br />';
  }

} else {

# fail result if no session token
  echo '<h3>Not logged in</h3>';
  echo '<p><a href="?action=login">Log In</a></p>';
}

# main function for curl requests
function apiRequest($url, $post=FALSE, $headers=array()) {
  $ch = curl_init($url);

  curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
  curl_setopt($ch, CURLOPT_USERAGENT, 'Linux useragent'); //change agent string

  if($post)
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post));

  $headers[] = 'Accept: application/json';

  # add access token to header
  if(session('access_token'))
    $headers[] = 'Authorization: Bearer ' . session('access_token');

  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

  $response = curl_exec($ch);
  return json_decode($response); //decode response
}

# array key existence
function get($key, $default=NULL) {
  return array_key_exists($key, $_GET) ? $_GET[$key] : $default;
}

# array key existence
function session($key, $default=NULL) {
  return array_key_exists($key, $_SESSION) ? $_SESSION[$key] : $default;
}
?>

Also make sure you add https