Recursively Zip Multiple Folders in PHP - recursion

Question

Following code is working fine, but only zipping one sub folder. My folder structure is like this.
MasterFolder
--example-folder
----TestFolder1
----TestFolder2
----TestFolder3
--example-file1.txt
--example-file2.txt
I need to zip the example-folder along with all the subfolders and files inside the example-folder and sub folders (TestFolder1, TestFolder2, TestFolder3, example-file1.txt, example-file2.txt) as well.
Here is my code.
create-zip-file.php
<?php
$zipArchive = new ZipArchive();
$zipFile = "./example-zip-file.zip";
if ($zipArchive->open($zipFile, ZipArchive::CREATE) !== TRUE) {
exit("Unable to open file.");
}
$folder = 'example-folder/';
createZip($zipArchive, $folder);
$zipArchive->close();
echo 'Zip file created.';
function createZip($zipArchive, $folder)
{
if (is_dir($folder)) {
if ($f = opendir($folder)) {
while (($file = readdir($f)) !== false) {
if (is_file($folder . $file)) {
if ($file != '' && $file != '.' && $file != '..') {
$zipArchive->addFile($folder . $file);
}
} else {
if (is_dir($folder . $file)) {
if ($file != '' && $file != '.' && $file != '..') {
$zipArchive->addEmptyDir($folder . $file);
$folder = $folder . $file . '/';
createZip($zipArchive, $folder);
}
}
}
}
closedir($f);
} else {
exit("Unable to open directory " . $folder);
}
} else {
exit($folder . " is not a directory.");
}
}
$filename = "example-zip-file.zip";
if (file_exists($filename)) {
$absoluteFilePath = __DIR__ . '/' . $filename;
header('Pragma: public');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Cache-Control: private', false);
header('Content-Type: application/zip');
header('Content-Disposition: attachment; filename="' . basename($filename) . '";');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . filesize($absoluteFilePath));
readfile($absoluteFilePath);
exit();
}
?>
index.php
<div class='container'>
<h2>Click Here to Download</h2>
<p>
Download
</p>
</div>