Zip files from checkbox and download

I am trying to make a list of files from a directory have them shown in a form via checkboxes. When the user checks files they want to download have them zipped and downloaded. I have basically no experience in php but have managed to make my list with checkboxes and a download script attached to the button that when it is selected should start the download but doesn't it does however clear the boxes.If possible I would also like to store a copy of this zip to a folder on the server in case the user would like to re-download in a certain amount of time. here is the code I have. Someone let me know where I went wrong and please explain any corrections to code as I am trying to learn as I do this little project.

 <?php
    $error = "";        //error holder
    if(isset($_POST['createzip'])){
        $post = $_POST;     
        $file_folder = "files/";    // folder to load files
        if(extension_loaded('zip')){    // Checking ZIP extension is available
            if(isset($post['files']) and count($post['files']) > 0){    // Checking files are selected
                $zip = new ZipArchive();            // Load zip library 
                $zip_name = time().".zip";          // Zip name
                if($zip->open($zip_name, ZIPARCHIVE::CREATE)!==TRUE){       // Opening zip file to load files
                    $error .=  "* Sorry ZIP creation failed at this time<br/>";
                }
                foreach($post['files'] as $file){               
                    $zip->addFile($file_folder.$file);          // Adding files into zip
                }
                $zip->close();
                if(file_exists($zip_name)){
                    // push to download the zip
                    header('Content-type: application/zip');
                    header('Content-Disposition: attachment; filename="'.$zip_name.'"');
                    readfile($zip_name);
                    // remove zip file is exists in temp path
                    unlink($zip_name);
                }

            }else
                $error .= "* Please select file to zip <br/>";
        }else
            $error .= "* You dont have ZIP extension<br/>";
    }
?>

Here's checkbox

<?php

if ($handle = opendir('./')) {
while (false !== ($entry = readdir($handle))) {
    if ($entry != "." && $entry != "..") {
        echo '<input type="checkbox" name="files[]" value=".$entry"/> '.$entry. '<br />';
    }
}
closedir($handle);
?>