函数参数没有传递给函数

Question

I'm trying to pass three variables arguments to function as following:

$productName = 'Product';
$details = 'size';
$existingCart = 'Nothing';
$newCart = $this->cartHandler($productName, $details, $existingCart);

Handler function:

private function cartHandler($productName, $details, $existingCart)
{
     return $details;
}

This will return nothing. Same result for returning $productName & $existingCart. These two functions are in same class.