Paypal结账：物品编号和金额缺失。

I have integrated PayPal in my WordPress site. All the required parameters are fetched and user is redirected to PayPal. The code is as follows:

        $fields = array(
            'item_name' => 'item_name_01',
            'item_number' => 'item_number_01',
            'cmd' => urlencode('_s-xclick'),
            'hosted_button_id' => urlencode('UYRL3AXNCTXCL'),
            'amount' => 100
        );

        foreach ($fields as $key => $value) {
                $fields_string .= $key . '=' . $value . '&';
            }
            rtrim($fields_string, '&');
            $url = 'https://www.sandbox.paypal.com/cgi-bin/webscr?' . $fields_string;
            header('Location: ' . $url);
            exit();

This works most of the times but sometimes, 'item_number' and 'amount' doesn't show up when redirected to PayPal. This behaviour is random and I can't exactly find when that happens and when it doesn't.

Any help is appreciated. Thanks in advance.