I need to add _ (underscore) to file zip file name am uploading

<form action="upload.php" enctype="multipart/form-data" method="post">
<input type="file" name="zipFile" accept="zip/*">
<input type="submit" name="Submit" value="Submit">
</form> 

$rand = rand(10,100);
$streplaceFileName = cleanSpecialCharacters($_FILES[‘zipFile’][‘name’]);
$zipFile = $rand."-".$streplaceFileName;$ds = DIRECTORY_SEPARATOR;
$storeFolder = ‘uploads/zip’;

if((!empty($_FILES)) && !empty($_FILES[‘zipFile’][‘name’])) {
if(preg_match(‘/[.](zip)$/’, $_FILES[‘zipFile’][‘name’])) {

$filename = $rand . "-" . $streplaceFileName;
$tempFile = $_FILES[‘zipFile’][‘tmp_name’];
$targetPath = $storeFolder . $ds;
$targetFile = $targetPath.$filename;
$check = move_uploaded_file($tempFile,$targetFile);
if($check) echo "ZipFile Uploaded Successfully!";
}
}

function cleanSpecialCharacters($string) {
$string = str_replace(‘ ‘, ‘-‘, $string); // Replaces all spaces with hyphens.
return preg_replace(‘/[^A-Za-z0-9.\-]/’, ", $string); // Removes special chars.
}