在php中创建发票PDF时出错

请帮助解决我的神话创建一个简单的发票详细php代码如下....我有两个表作为"a_sales"answers"a_sales_detail"，我尝试创建一个发票pdf。现在它根本不会显示.....

<div class="row">
<div class="col-lg-6 col-md-6 col-sm-6">
<div class="panel panel-default">
 <div class="panel-heading">Item Information</div>
 <div class="panel-body">
 <?php
 $sql = "SELECT DISTINCT ";
 "`a_sales_detail`.`Sales_Quantity` AS `OrderQTY`,";
 "`a_sales_detail`.`Stock_Item` AS `Item`,";
 "`a_sales_detail`.`Item_Description` AS `Description`,";
 "`a_sales_detail`.`Sales_Price` AS `UnitPrice`";
 "`a_sales_detail`.`Sales_Total_Amount ` AS `Subtotal`";
 " FROM ` a_sales ` JOIN `a_sales_detail ` ON (`a_sales`.`Sales_Number` = `a_sales_detail`.`Sales_Number`)";
 echo $dbhelper->ExecuteHtml($sql, array("fieldcaption" => TRUE, "tablename" => array("a_sales", "a_sales_detail"))); 
?>