PHP中的ZipArchive不会解压缩文件

Question

Here is the code I use

<?php
$zip = new ZipArchive;
     $res = $zip->open('file.zip');
     if ($res === TRUE) {
         $zip->extractTo('myextractdir/');
         $zip->close();
         echo 'Extracted';
     } else {
         echo 'failed';
     }
?>

It just outputs Extracted, but no files show up. The file is a zip file..