How can PHP be used to create a backup system using Gmail?

```php
<?php
require_once __DIR__ . '/vendor/autoload.php';

$client = new Google_Client();
$client->setAuthConfig('credentials.json');
$client->addScope(Google_Service_Gmail::GMAIL_SEND);

$service = new Google_Service_Gmail($client);

$file_path = '/path/to/backup.zip';
$file_name = 'backup.zip';

$email = new Google_Service_Gmail_Message();
$email->setRaw(strtr(base64_encode("From: sender@gmail.com\r\nTo: recipient@gmail.com\r\nSubject: Backup\r\nMIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=boundary_string\r\n--boundary_string\r\nContent-Type: text/plain; charset='UTF-8'\r\n\r\nPlease find the backup attached.\r\n--boundary_string\r\nContent-Type: application/zip; name=$file_name\r\nContent-Disposition: attachment; filename=$file_name\r\nContent-Transfer-Encoding: base64\r\n\r\n" . base64_encode(file_get_contents($file_path)) . "\r\n--boundary_string--"), '+/', '-_'));
$service->users_messages->send('me', $email);
```

Make sure to replace 'sender@gmail.com' and 'recipient@gmail.com' with your own email addresses, and 'credentials.json' with your own Google API credentials file. Also, replace '/path/to/backup.zip' with the actual path to your backup file.