在根目录下用php压缩

I need to create a zip file in php, including in it some doc and pdf files. I use a function found on the web:

function create_zip($files = array(),$destination = '',$overwrite = true) {
    //if the zip file already exists and overwrite is false, return false
    if(file_exists($destination) && !$overwrite) { return false; }
    //vars
    $valid_files = array();
    //if files were passed in...
    if(is_array($files)) {
        //cycle through each file
        foreach($files as $file) {
            //make sure the file exists
            if(file_exists($file)) {
                $valid_files[] = $file;
            }
        }
    }
    //if we have good files...
    if(count($valid_files)) {
        $zip = new ZipArchive();
        if($zip->open($destination,$overwrite ? ZIPARCHIVE::OVERWRITE : ZIPARCHIVE::CREATE) !== true) {
            return false;
        }

        foreach($valid_files as $file)
            $zip->addFile($file,$file);

        $zip->close();

        return file_exists($destination);
    }
    else
        return false;
}

The problem is that if the file path to include into the zip is path/to/file/filename.pdf when I download and open the zip i get filename.pdf contained into path/to/file folders. How can I do to put all the files in the root of the zip?

Use the second parameter of ZipArchive::addFile() to specify under which file name the file appears in the archive. basename($file) is a good candidate for that.