Cannot upload file to Google Drive using API for service account - php

Question

To upload files to Google Drive using service account, I have written the following code which is from this link: https://developers.google.com/drive/web/service-accounts
My code:
require_once "google-api-php-client/src/Google_Client.php";
require_once "google-api-php-client/src/contrib/Google_DriveService.php";
require_once "google-api-php-client/src/contrib/Google_Oauth2Service.php";
session_start();
$DRIVE_SCOPE = 'https://www.googleapis.com/auth/drive';
$SERVICE_ACCOUNT_EMAIL = '<some-id>#developer.gserviceaccount.com';
$SERVICE_ACCOUNT_PKCS12_FILE_PATH = '/path/to/<public_key_fingerprint>-privatekey.p12';
/**
* Build and returns a Drive service object authorized with the service accounts.
*
* #return Google_DriveService service object.
*/
function buildService() {
$key = file_get_contents($SERVICE_ACCOUNT_PKCS12_FILE_PATH);
$auth = new Google_AssertionCredentials(
SERVICE_ACCOUNT_EMAIL,
array(DRIVE_SCOPE),
$key);
$client = new Google_Client();
$client->setUseObjects(true);
$client->setAssertionCredentials($auth);
return new Google_DriveService($client);
}
function uploadFile($service, $mime, $src) {
//Insert a file
$file = new Google_DriveFile();
$file->setMimeType($mime);
$data = file_get_contents($src);
try {
//ERROR HERE: cannot insert (upload) file
$createdFile = $service->files->insert($file,
array(
'data' => $data,
'mimeType' => $mime,
'convert' => true,
)
);
return $createdFile;
} catch (Exception $e) {
print "An error occurred: " . $e->getMessage();
}
}
$service = buildService();
When I call the buildService() method, it gives me an error:
Catchable fatal error: Object of class Google_AssertionCredentials could not be converted to string in /opt/lampp/htdocs/ajaxGoogleDrive/google-api-php-client/src/auth/Google_OAuth2.php on line 197
Where am I going wrong?