Inserting Google Calendar Entries with Service Account

I figured this out. Since I don't see any complete examples of using service accounts with API v3, I'm just going to post my complete solution for reference. There are a few of things that you need to do in addition to implementing the code, however:

1) You need to go to the Google Developer's console and mark your account as a 'service account'. This will differentiate it from a web application. The important difference is that nobody will be prompted to log in to their account before the events are added since the account will belong to your application, not an end user. For more information see this article, starting on page 5.

2) You need to create a public/private key pair. From the developer's console, click on Credentials. Under you service account, click on 'Generate new P12 key'. You'll need to store this somewhere. That file location becomes the $key_file_location variable string in the code below.

3) Also from the developer's console, you need to enable the Calendar API. From your project, on the left margin you'll see APIs. Select that and find the Calendar API. Click it, accept the terms of service, and verify that it is now displayed under Enabled APIs with a status of On

4) In Google Calendar that you want to add events to, under settings, click Calendar Settings then on 'Share this Calendar' at the top. Under 'Share with specific people' in the 'Person' field, paste in the email address from the service account credentials. Change the permission settings to 'Make changes to events'. Don't forget to save the change.

Then, implement this code somewhere.

Comment if something is confusing or omitted. Good luck!

<?php

function calendarize ($title, $desc, $ev_date, $cal_id) {

    session_start();

    /************************************************
    Make an API request authenticated with a service
    account.
    ************************************************/
    set_include_path( '../google-api-php-client/src/');

    require_once 'Google/Client.php';
    require_once 'Google/Service/Calendar.php';

    //obviously, insert your own credentials from the service account in the Google Developer's console
    $client_id = '843319906820-xxxxxxxxxxxxxxxxxxxdcqal54p1he6.apps.googleusercontent.com';
    $service_account_name = '843319906820-xxxxxxxxxxxxxxxxxxxdcqal54p1he6@developer.gserviceaccount.com';
    $key_file_location = '../google-api-php-client/calendar-xxxxxxxxxxxx.p12';

    if (!strlen($service_account_name) || !strlen($key_file_location))
        echo missingServiceAccountDetailsWarning();

    $client = new Google_Client();
    $client->setApplicationName("Whatever the name of your app is");

    if (isset($_SESSION['service_token'])) {
        $client->setAccessToken($_SESSION['service_token']);
    }

    $key = file_get_contents($key_file_location);
    $cred = new Google_Auth_AssertionCredentials(
        $service_account_name, 
        array('https://www.googleapis.com/auth/calendar'), 
        $key
    );
    $client->setAssertionCredentials($cred);
    if($client->getAuth()->isAccessTokenExpired()) {
        $client->getAuth()->refreshTokenWithAssertion($cred);
    }
    $_SESSION['service_token'] = $client->getAccessToken();

    $calendarService = new Google_Service_Calendar($client);
    $calendarList = $calendarService->calendarList;

    //Set the Event data
    $event = new Google_Service_Calendar_Event();
    $event->setSummary($title);
    $event->setDescription($desc);

    $start = new Google_Service_Calendar_EventDateTime();
    $start->setDateTime($ev_date);
    $event->setStart($start);

    $end = new Google_Service_Calendar_EventDateTime();
    $end->setDateTime($ev_date);
    $event->setEnd($end);

    $createdEvent = $calendarService->events->insert($cal_id, $event);

    echo $createdEvent->getId();
} 

?>

Some helpful resources:
Github example for service accounts
Google Developers Console for inserting events in API v3
Using OAuth 2.0 to Access Google APIs

Here is the Google API Client 2.0 way to do

Follow Alex's Answer Go Google Developer's console to create Service Account, but create credential in JSON

Follow Google API Client Upgrade Guide, install by composer and include in the way of

 require_once 'vendor/autoload.php';

Follow Alex's Answer to go to Google Calendar to share the calendar with Service Account ID

[email protected] 

Take Ref. to below coding and the most important one is the calendar id should be the email of calendar creator, but not primary

<?php


require_once __DIR__.'/vendor/autoload.php';
   session_start();     


$client = new Google_Client();
$application_creds = __DIR__.'/secret.json';  //the Service Account generated cred in JSON
$credentials_file = file_exists($application_creds) ? $application_creds : false;
define("APP_NAME","Google Calendar API PHP");   //whatever
$client->setAuthConfig($credentials_file);
$client->setApplicationName(APP_NAME);
$client->addScope(Google_Service_Calendar::CALENDAR);
$client->addScope(Google_Service_Calendar::CALENDAR_READONLY);

//Setting Complete

//Go Google Calendar to set "Share with ..."  Created in Service Account ([email protected])

//Example of Use of API
$service = new Google_Service_Calendar($client);  


$calendarId = '[email protected]';   //NOT primary!! , but the email of calendar creator that you want to view
$optParams = array(
  'maxResults' => 10,
  'orderBy' => 'startTime',
  'singleEvents' => TRUE,
  'timeMin' => date('c'),
);
$results = $service->events->listEvents($calendarId, $optParams);

if (count($results->getItems()) == 0) {
  print "No upcoming events found.\n";
} else {
  echo "Upcoming events:";
  echo "<hr>";
  echo "<table>";
  foreach ($results->getItems() as $event) {
    $start = $event->start->dateTime;
    if (empty($start)) {
      $start = $event->start->date;
    }
    echo "<tr>";
    echo"<td>".$event->getSummary()."</td>";
    echo"<td>".$start."</td>";
    echo "</tr>";
  }
  echo "</table>";
}