Php Paypal金额不能正常工作只显示项目价格

Question

这是我的paypal代码

$data=array(
'merchant_email'=>'test@test.com',
'product_name'=> implode(',',$prdname) ,
'amount'=> 300 ,        
'currency_code'=>'USD',
'thanks_page'=>"http://".$_SERVER['HTTP_HOST']."/dev/x/?remove_cart=1",
'notify_url'=>"http://".$_SERVER['HTTP_HOST']."/dev/x/",
'cancel_url'=>"http://".$_SERVER['HTTP_HOST']."/dev/x/",
'paypal_mode'=>true,
);

这是我的paypal表格

function infotutsPaypal( $data) {
 $grandtotal = ($_SESSION['grandtotal'] + $_SESSION['smethod']);

define( 'SSL_URL', 'https://www.paypal.com/cgi-bin/webscr' );
 define( 'SSL_SAND_URL', 'https://www.sandbox.paypal.com/cgi-bin/webscr' );

$action = '';
 //Is this a test transaction?
 $action = ($data['paypal_mode']) ? SSL_SAND_URL : SSL_URL;

$form = '';

$form .= '<form name="frm_payment_method" action="' . $action . '" method="post">';
 $form .= '<input type="hidden" name="business" value="' . $data['merchant_email'] . '" />';
 // Instant Payment Notification & Return Page Details /
 $form .= '<input type="hidden" name="notify_url" value="' . $data['notify_url'] . '" />';
 $form .= '<input type="hidden" name="cancel_return" value="' . $data['cancel_url'] . '" />';
 $form .= '<input type="hidden" name="return" value="' . $data['thanks_page'] . '" />';
 $form .= '<input type="hidden" name="rm" value="2" />';
 // Configures Basic Checkout Fields -->
 $form .= '<input type="hidden" name="lc" value="US" />';

 $form .= '<input type="hidden" name="quantity" value="'.$_SESSION['quantity'].'">';
 // <input type="hidden" name="custom" value="localhost" />-->
 $form .= '<input type="hidden" name="currency_code" value="USD" />';
 $form .= '<input type="hidden" name="amount" value="'. $grandtotal.'">';
 $form .= '<input type="hidden" name="page_style" value="paypal" />';
 $form .= '<input type="hidden" name="charset" value="utf-8" />';
 $form .= '<input type="hidden" name="item_name" value="' . $data['product_name'] . '" />';
 $form .= '<input type="hidden" value="_xclick" name="cmd"/>'; 
 $form .= '<script>';
 $form .= 'setTimeout("document.frm_payment_method.submit()", 2);';
 $form .= '</script>';
 $form .= '</form>';
 return $form;
 }

我只得到物品价格正确传递，总金额显示一些随机数，请建议如何解决这个问题，谢谢adavce。