How about using "About: get" of Drive API? When this is reflected to the PHP script using googleapis for PHP, it becomes as follows.
Sample script:
$service = new Google_Service_Drive($client); // Please use your "$client".
$res = $service->about->get(['fields' => 'storageQuota']);
$storageQuota = $res -> getStorageQuota();
print_r($storageQuota);
// When you want to retrieve the value of "usage` of "storageQuota", you can do it as follows.
$usage = $storageQuota -> getUsage();
echo $usage;
When above script is run, the following value is obtained.
{
"limit": "###",
"usage": "###",
"usageInDrive": "###",
"usageInDriveTrash": "###"
}
Reference: