ได้นำ code ปุ่ม เพิ่ม ลบ จำนวนสินค้า แต่ บันทัดแรกไม่สามารถกดได้

<?php
ob_start();
session_start();
	
if(!isset($_SESSION["intLine"]))
{
	if(isset($_POST["txtProductID"]))
	{
		 $_SESSION["intLine"] = 0;
		 $_SESSION["strProductID"][0] = $_POST["txtProductID"];
		 $_SESSION["strQuanlity"][0] = $_POST["txtQuanlity"];

		 header("location:product_summary.php");
	}
}
else
{
	
	$key = array_search($_POST["txtProductID"], $_SESSION["strProductID"]);
	if((string)$key != "")
	{
			if(isset($_POST['plus']))
				{
					$_SESSION["strQuanlity"][$key] = $_SESSION["strQuanlity"][$key] + $_POST["txtQuanlity"];
				}
			else  if (isset($_POST['minus']))
				{
					$_SESSION["strQuanlity"][$key] = $_SESSION["strQuanlity"][$key] - $_POST["txtQuanlity"];
				}	

	}

	else
	{

		 $_SESSION["intLine"] = $_SESSION["intLine"] + 1;	
		 $intNewLine = $_SESSION["intLine"];
		 $_SESSION["strProductID"][$intNewLine] = $_POST["txtProductID"];
		 $_SESSION["strQuanlity"][$intNewLine] = $_POST["txtQuanlity"];
	}
	 header("location:product_summary.php");

}
?>

<form action="order-plus.php" method="post"> 
                                <input name="txtP[]" type="hidden" id="txtP[]" value="<?=$i;?>">
                                <input type="text" id="txtQua<?=$strNum;?>" name="txtQua[]" class="span1 form-control input-number" placeholder="1" min="1" max="100" value="<?=$_SESSION["strQuanlity"]["$i"];?>" maxlength="6" onChange="checknum(this.value,'txtQua<?=$strNum;?>');" disabled  >
                    
                                <input type="hidden" name="txtProductID" value="<?php echo $objResult["ProductID"];?>" size="2">
                                <input type="hidden" name="txtQuanlity" value="<?php echo $objResult["Packing"];?>" class="span1" placeholder="Qty."/>
                                <button class=" btn txtQua-right-plus btn btn-success btn-number" type="submit" name="minus[0]" id="minus" value="minus"><span class="glyphicon glyphicon-minus"><i class="icon-minus"></i></span></button>
                                <button class=" btn txtQua-right-plus btn btn-success btn-number" type="submit" name="plus[1]" id="plus" value="plus"><span class="glyphicon glyphicon-plus"><i class="icon-plus"></i></span></button>
                 				<button class="btn btn-danger" type="button"> <a href="javascript:if(confirm('ลบรายการสินค้า')==true){window.location='<?=$_SERVER['PHP_SELF'];?>?action=Del&P=<?=$i;?>';}"><i class="icon-remove icon-white"></i></a></button>   
                   	
						</form>