This script will zip multiple files and  force download created zip file. This function required Zip Archive enable in your server.

<?php
function zipFilesDownload($file_names,$archive_file_name,$file_path){
$zip = new ZipArchive();
  if ($zip->open($archive_file_name, ZIPARCHIVE::CREATE )!==TRUE) {
  exit("cannot open <$archive_file_name>\n");
  }
  foreach($file_names as $files){
  $zip->addFile($file_path.$files,$files);
  }
$zip->close();

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

$fileNames=array('files/file1.docx','files/file1.pdf');
$zip_file_name='myFile.zip';
$file_path=dirname(__FILE__).'/';

zipFilesDownload($fileNames,$zip_file_name,$file_path);

?>

Also Check:

Unzip Files in Web server

<?php
$zip = zip_open("moooredale.zip");
if ($zip) {
   while ($zip_entry = zip_read($zip)) {
     $fp = fopen(zip_entry_name($zip_entry), "w");
     if (zip_entry_open($zip, $zip_entry, "r")) {
       $buf = zip_entry_read($zip_entry, zip_entry_filesize($zip_entry));
       fwrite($fp,"$buf");
       zip_entry_close($zip_entry);
       fclose($fp);
     }
  }
zip_close($zip);
}
?>