php ZipArchive setPassword is not work

$zip = new ZipArchive();
$code = $zip->open('myzip.zip', ZipArchive::CREATE | ZipArchive::OVERWRITE);
if ($code === true)
    echo 'zip opened<br>';
else
    echo $code.'<br>';
$zip->addFile('somefile');

$code = $zip->setPassword('secret');

if ($code === true)
    echo 'password set<br>';
else
    var_dump($code);

$code = $zip->close();

on the filesystem, the myzip.zip is created with somefile inside and $code is true but the zip file is not password protected ...
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