index.php

<?php

/**
 * @file
 *
 * Reads data from a Google Spreadsheet that needs authentication.
 */

require 'vendor/autoload.php';

/**
 * Set here the full path to the private key .json file obtained when you
 * created the service account. Notice that this path must be readable by
 * this script.
 */
$service_account_file = '';

/**
 * This is the long string that identifies the spreadsheet. Pick it up from
 * the spreadsheet's URL and paste it below.
 */
$spreadsheet_id = '';

/**
 * This is the range that you want to extract out of the spreadsheet. It uses
 * A1 notation. For example, if you want a whole sheet of the spreadsheet, then
 * set here the sheet name.
 *
 * @see https://developers.google.com/sheets/api/guides/concepts#a1_notation
 */
$spreadsheet_range = '';

putenv('GOOGLE_APPLICATION_CREDENTIALS=' . $service_account_file);
$client = new Google_Client();
$client->useApplicationDefaultCredentials();
$client->addScope(Google_Service_Sheets::SPREADSHEETS_READONLY);
$service = new Google_Service_Sheets($client);

$result = $service->spreadsheets_values->get($spreadsheet_id, $spreadsheet_range);
var_dump($result->getValues());