Create a zip file and download it

// http headers for zip downloads
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"".$filename."\"");
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".filesize($filepath.$filename));
ob_end_flush();
@readfile($filepath.$filename);

I found this soludtion here and it work for me

Add Content-length header describing size of zip file in bytes.

header("Content-type: application/zip"); 
header("Content-Disposition: attachment; filename=$archive_file_name");
header("Content-length: " . filesize($archive_file_name));
header("Pragma: no-cache"); 
header("Expires: 0"); 
readfile("$archive_file_name");

Also make sure that there is absolutely no white space before <? and after ?>. I see a space here:

↓

 <?php
$file_names = array('iMUST Operating Manual V1.3a.pdf','iMUST Product Information Sheet.pdf');

$zip = new ZipArchive;
$tmp_file = 'assets/myzip.zip';
    if ($zip->open($tmp_file,  ZipArchive::CREATE)) {
        $zip->addFile('folder/bootstrap.js', 'bootstrap.js');
        $zip->addFile('folder/bootstrap.min.js', 'bootstrap.min.js');
        $zip->close();
        echo 'Archive created!';
        header('Content-disposition: attachment; filename=files.zip');
        header('Content-type: application/zip');
        readfile($tmp_file);
   } else {
       echo 'Failed!';
   }