$zip = new ZipArchive();
$filename = "gs://my_bucket/new_zip_file.zip";
$toadd = 'gs://my_bucket/MakeaZip.txt';

if (file_exists($toadd)) {
    echo "The file $toadd exists<br>";
} else {
    echo "The file $toadd does not exist<br>";
}

if (is_writable('gs://my_bucket')) {
    echo 'Folder is writable<br>';
} else {
    echo 'Folder not writable<br>';
}

if ($zip->open($filename, ZipArchive::CREATE)!==TRUE) {
    echo 'Can not open '.$filename.'<br>';
} else {
    echo 'Can open '.$filename.'<br>';
}

$content = file_get_contents($toadd);
$zip->addFromString(pathinfo ( $toadd, PATHINFO_BASENAME), $content);

echo "numfiles: " . $zip->numFiles . '<br>';
echo "status:" . $zip->status . '<br>';
echo 'GetStatusString: '.$zip->getStatusString() . '<br>';
$ret = $zip->close();
echo 'Closed with: ' . ($ret ? "true" : "false") . '<br>';
echo 'GetStatusString: '.$zip->getStatusString() . '<br>';