$templateProcessor = new TemplateProcessor('sample.docx');

$firstname = 'John';
$middlename = 'Mc';
$lastname = 'Doe';

// current
$templateProcessor->setValues([
    "firstname" => $firstname,
    "middlename" => $middlename,
    "lastname" => $lastname,
]);

// assuming we want to change the only the "first" name variables:
// workaround
$templateProcessor->setValue("firstname", $firstname, 1);
$templateProcessor->setValue("middlename", $middlename, 1);
$templateProcessor->setValue("lastname", $lastname, 1);

// improvement
$templateProcessor->setValues([
    "firstname" => $firstname,
    "middlename" => $middlename,
    "lastname" => $lastname,
], 1);