$image = file_get_contents('http://url.com/image.jpg');
file_put_contents('tmp/15.jpg', $image);

$image = file_get_contents('http://url.com/image.jpg');
file_put_contents('tmp/16.jpg', $image);

$image = file_get_contents('http://url.com/image.jpg');
file_put_contents('tmp/17.jpg', $image);


$files = array(
    'tmp/15.jpg',
    'tmp/16.jpg',
    'tmp/17.jpg'
);

$valid_files = array();
if(is_array($files)) {
    foreach($files as $file) {
        if(file_exists($file)) {
            $valid_files[] = $file;
        }
    }
}

if(count($valid_files > 0)){
    $zip = new ZipArchive();
    $zip_name = "images.zip";
    if($zip->open($zip_name, ZIPARCHIVE::CREATE)!==TRUE){
        $error .= "* Sorry ZIP creation failed at this time";
    }

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

    $zip->close();
    if(file_exists($zip_name)){

        header("Pragma: public");
        header("Expires: 0");
        header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
        header("Cache-Control: private",false);
        header('Content-type: application/zip');
        header('Content-Disposition: attachment; filename="'.$zip_name.'"');
        readfile($zip_name);

        unlink($zip_name);
    }

} else {
    echo "No valid files to zip";
    exit;
}