怎么用php向服务器已存在的zip文件中增加一个文件

加一个zip模块进去
使用方式如下：
$zip = new ZipArchive();
$filename = "./test112.zip";
if ($zip->open($filename, ZIPARCHIVE::CREATE)!==TRUE) {
exit("cannot open <$filename> ");
}
$zip->addFromString("testfilephp.txt" . time(), "#1 This is a test string added as testfilephp.txt. ");
$zip->addFromString("testfilephp2.txt" . time(), "#2 This is a test string added as testfilephp2.txt. ");
$zip->addFile($thisdir . "/too.php","/testfromfile.php");
echo "numfiles: " . $zip->numFiles . " ";
echo "status:" . $zip->status . " ";
$zip->close();
?>