A simple PHP implementation of Foursquare's OAuth 2.0 flow. Useful for quickly getting a test app up and running.

foursquare_oauth.php

<?php

/**
 * This is a quick and dirty implementation of Foursquare's OAuth 2.0 flow. It handles the 
 * auth redirect and code exchange, and spits out a user access token.
 *
 * See https://developer.foursquare.com/overview/auth.html for more info.
 *
 * @author Ken Mickles
 */

// copy these from your Foursquare app settings (https://foursquare.com/developers/apps)
$client_id = '';
$client_secret = '';

// make sure this URL is set as a redirect URI
$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'];

// if we've got a code, the user just came back from authorizing the app
// next, we exchange the code for an access token
if ( isset($_GET['code']) ) {
  $url = 'https://foursquare.com/oauth2/access_token?client_id=' . $client_id . '&client_secret=' . $client_secret . '&grant_type=authorization_code&redirect_uri=' . $redirect_uri . '&code=' . $_GET['code'];

  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1); 
  $result = curl_exec($ch);
  curl_close($ch);
  
  $data = json_decode($result, 1);
  
  if ( isset($data['access_token']) ) {
    echo "Your access token is: <b>{$data['access_token']}</b>";
  }
  else {
    echo "Something went wrong: <pre>{$result}</pre>";
  }
}
// redirect to the Foursquare auth dialog
else {
  header('Location: https://foursquare.com/oauth2/authenticate?client_id=' . $client_id . '&response_type=code&redirect_uri=' . $redirect_uri);
}