Here's an example of how to get your oauth2.0 access token and get a list of your scheduled meetings.
I adapted it from this example: http://stackoverflow.com/questions/9273576/gotowebinar-api-php. Please give that guy full credit.  I just changed a few lines of code.


gotoMeetingClass.php:
<?php

define('GOTO_MEETING_API_KEY','Your api key goes here');

class OAuth_En{

protected $_accessToken;
protected $_userId;
protected $_organizerKey;
protected $_refreshToken;
protected $_expiresIn;

public function getAccessToken(){
    return $this->_accessToken;
}

public function setAccessToken($token){
    $this->_accessToken = $token;
}

public function getUserId(){
    return $this->_userId;
}

public function setUserId($id){
    $this->_userId = $id;
}   

public function getOrganizerKey(){
    return $this->_organizerKey;
}

public function setOrganizerKey($key){
    $this->_organizerKey = $key;
}

public function getRefreshToken(){
    return $this->_refreshToken;
}

public function setRefreshToken($token){
    $this->_refreshToken = $token;
}

public function getExpiresIn(){
    return $this->_expiresIn;
}

public function setExpiresIn($expiresIn){
    $this->_expiresIn = $expiresIn;
}   


}

class OAuth_Db{
function getToken(){

}       
}

class myOAuth{
protected $_redirectUrl;
protected $_OAuthEnObj;
protected $_curlHeader = array();
protected $_apiResponse;
protected $_apiError;
protected $_apiErrorCode;
protected $_apiRequestUrl;
protected $_apiResponseKey;
protected $_accessTokenUrl;
protected $_meetingId;
protected $_registrantInfo = array();
protected $_apiRequestType;
protected $_apiPostData;

public function __construct(OAuth_En $oAuthEn){
    $this->_OAuthEnObj = $oAuthEn;  
}

public function getOAuthEntityClone(){
    return clone $this->_OAuthEnObj;    
}

public function getMeetingId(){
    return $this->_meetingId;
}

public function setMeetingId($id){
    $id = (int)$id;
    $this->_meetingId = empty($id) ? 0 : $id;
}

public function setApiErrorCode($code){
    $this->_apiErrorCode = $code;   
}

public function getApiErrorCode(){
    return $this->_apiErrorCode;    
}   

public function getApiAuthorizationUrl(){
    return 'https://api.citrixonline.com/oauth/authorize?client_id='.GOTO_MEETING_API_KEY.'&redirect_uri='.$this->getRedirectUrl(); 
}

public function getApiKey(){
    return  GOTO_MEETING_API_KEY;
}

public function getApiRequestUrl(){
    return  $this->_apiRequestUrl;
}

public function setApiRequestUrl($url){
    $this->_apiRequestUrl = $url;
}

public function setRedirectUrl($url){
    $this->_redirectUrl = urlencode($url);  
}

public function getRedirectUrl(){
    return $this->_redirectUrl; 
}

public function setCurlHeader($header){
    $this->_curlHeader = $header;   
}

public function getCurlHeader(){
    return $this->_curlHeader;  
} 

public function setApiResponseKey($key){
    $this->_apiResponseKey = $key;
}

public function getApiResponseKey(){
    return $this->_apiResponseKey;
}

public function setRegistrantInfo($arrInfo){
    $this->_registrantInfo = $arrInfo;  
}

public function getRegistrantInfo(){
    return $this->_registrantInfo;  
}

public function authorizeUsingResponseKey($responseKey){
    $this->setApiResponseKey($responseKey);
    $this->setApiTokenUsingResponseKey();
}

protected function setAccessTokenUrl(){
    $url = 'https://api.citrixonline.com/oauth/access_token?grant_type=authorization_code&code={responseKey}&client_id={api_key}';
    $url = str_replace('{api_key}', $this->getApiKey(), $url);
    $url = str_replace('{responseKey}', $this->getApiResponseKey(), $url);
    $this->_accessTokenUrl = $url;
}

protected function getAccessTokenUrl(){
    return $this->_accessTokenUrl;  
}

protected function resetApiError(){
    $this->_apiError = '';  
}

public function setApiTokenUsingResponseKey(){
    //set the access token url
    $this->setAccessTokenUrl();

    //set the url where api should go for request
    $this->setApiRequestUrl($this->getAccessTokenUrl());

    //make request
    $this->makeApiRequest();

    if($this->hasApiError()){
        echo $this->getApiError();
    }else{
        //if api does not have any error set the token
        echo $this->getResponseData();
        $responseData = json_decode($this->getResponseData());
        $this->_OAuthEnObj->setAccessToken($responseData->access_token);
        $this->_OAuthEnObj->setOrganizerKey($responseData->organizer_key);
        $this->_OAuthEnObj->setRefreshToken($responseData->refresh_token);
        $this->_OAuthEnObj->setExpiresIn($responseData->expires_in);
    }
}

function hasApiError(){
    return $this->getApiError() ? 1 : 0;
}

function getApiError(){
    return $this->_apiError;
}

function setApiError($errors){
    return $this->_apiError = $errors;
}

function getApiRequestType(){
    return $this->_apiRequestType;
}

function setApiRequestType($type){
    return $this->_apiRequestType = $type;
}   

function getResponseData(){
    return $this->_apiResponse;
}

function setApiPostData($data){
    return $this->_apiPostData = $data;
}   

function getApiPostData(){
    return $this->_apiPostData;
}   

function makeApiRequest(){
    $header = array();

    $this->getApiRequestUrl();
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE); 
    curl_setopt($ch, CURLOPT_URL, $this->getApiRequestUrl());
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    if($this->getApiRequestType()=='POST'){
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $this->getApiPostData());  
    }

    if($this->getCurlHeader()){
        $headers = $this->getCurlHeader();
    }else{
        $headers = array(
                "HTTP/1.1",
                "Content-type: application/json",
                "Accept: application/json",
                "Authorization: OAuth oauth_token=".$this->_OAuthEnObj->getAccessToken()
            );  
    }

    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers); 

    $data = curl_exec($ch);
    $validResponseCodes = array(200,201,409);
    $responseCode = curl_getinfo($ch, CURLINFO_HTTP_CODE); 

    $this->resetApiError();

    if (curl_errno($ch)) {
        $this->setApiError(array(curl_error($ch)));
    } elseif(!in_array($responseCode, $validResponseCodes)){
        if($this->isJsonString($data)){
            $data = json_decode($data);
        }

        $this->setApiError($data);
        $this->setApiErrorCode($responseCode);
    }else {
        $this->_apiResponse = $data;
        $_SESSION['gotoApiResponse'] = $this->getResponseData();
        curl_close($ch);
    }
}

function isAuthorizationRequiredAgain(){
    $arrAuthorizationRequiredCodes = array(400,401,403,500);
    $isAuthRequired = 0;
    $error = $this->getApiError();
    $responseCode = $this->getApiErrorCode();

    //we might have to add more exception in this condition
    if(in_array($responseCode, $arrAuthorizationRequiredCodes)){
        if($responseCode==400 && is_object($error)){    //because for 400 error sometime one needs to authenticate again
            foreach($error as $single){
                $pos = strpos($single,'Authorization');
                if($pos!==false){
                    $isAuthRequired = 1;
                }
            }
        }else{
            $isAuthRequired = 1;    
        }
    }

    return $isAuthRequired;
}

function getMeetings(){
    $url = 'https://api.citrixonline.com/G2M/rest/meetings?scheduled=true&oauth_token='.$this->_OAuthEnObj->getAccessToken();        
    $this->setApiRequestUrl($url);
    $this->setApiRequestType('GET');
    $this->makeApiRequest();

    if($this->hasApiError()){
        return null;    
    }

    $meetings = json_decode($this->getResponseData());
    $test = $this->_OAuthEnObj->getAccessToken();
    return $meetings;
}

function getAuthURL(){
    $url = 'https://api.citrixonline.com/G2M/rest/meetings?scheduled=true&oauth_token='.$this->_OAuthEnObj->getAccessToken();
    return $url;
}

function isJsonString($string){
    $isJson = 0;
    $decodedString = json_decode($string);
    if(is_array($decodedString) || is_object($decodedString))
        $isJson = 1;    

    return $isJson;
}
}

-----------------------------
Authorize.php:
<?php
include_once "gotoMeetingClass.php";
define('REDIRECT_URL_AFTER_AUTHENTICATION','http://example.com/authorize.php'); //this is the url where your get token code would be written.

session_start();
$obj = new OAuth_En();

$oauth = new myOAuth($obj);


if(!isset($_GET['code'])){  
    goForAuthorization();
}else{  //when user authenticates and redirect back to application redirect url, get the token
    $oauth->authorizeUsingResponseKey($_GET['code']);
    if(!$oauth->hasApiError()){
        $objOAuthEn = $oauth->getOAuthEntityClone();
        $_SESSION['oauthEn'] = serialize($objOAuthEn);
        header('Location: get-all-meetings.php');       
    }
}

//this function has been used for getting the key using which we can get the access token
function goForAuthorization(){
    global $oauth;
    $oauth->setRedirectUrl(REDIRECT_URL_AFTER_AUTHENTICATION);
    $url = $oauth->getApiAuthorizationUrl();
    header('Location: '.$url);
}



--------------------

gotoMeetingClass.php:
<?php

include_once "gotoMeetingClass.php";
session_start();
$obj = unserialize($_SESSION['oauthEn']);

/*
this can be used to fetch the stored access token key and organizer key from database and use it without asking the authetication from user again

$obj = new OAuth_En();
$obj->setAccessToken('token');
$obj->setOrganizerKey('organizer key');
*/

$oauth = new myOAuth($obj);
$meetings = $oauth->getMeetings();

echo '<pre>';
if(!$oauth->hasApiError()){
    print_r($meetings); 
}else{
    print_r($oauth->getApiError());
    print_r($oauth->getApiErrorCode());
    print_r($oauth->getAuthURL()); 
}
exit;

/*$webinars = $oauth->getUpcomingWebinars();

if(!$oauth->hasApiError()){
    print_r($webinars); 
}else{
    print_r($oauth->getApiError()); 
}

exit;
$registrantInfo = array(
    "firstName"=>"ashish",
    "lastName"=>"mehta",
    "email"=>"test@test.com",
);

$oauth->setWebinarId(525120321);
$oauth->setRegistrantInfo($registrantInfo);

$res = $oauth->createRegistrant();
echo $oauth->getApiErrorCode();
if(!$oauth->hasApiError()){
    print_r($res);  
}else{
    echo 'error';
    print_r($oauth->getApiError()); 
}

exit;
$oauth->setWebinarId(525120321);
$webinar = $oauth->getWebinar();

if(!$oauth->hasApiError()){
    print_r($webinar);  

}else{
    print_r($oauth->getApiError()); 
    echo $oauth->getApiErrorCode();
}
*/