使用PHP插入空值的数据

here order_ID ,Bill may b null,a customer dont need to fill in the form ,,,here is the php code ,,,,,it is not working.....i dont know how to insert tuple with null values into table by using php.i cant find the errors.

create table Orders(
    Order_ID number(10) primary key,
    Cust_id number(5),
    Order_date date,
    Bill number(5,2),
    CONSTRAINT fk_cust  FOREIGN KEY (Cust_ID) REFERENCES Customer(Cust_ID)
    );


CREATE SEQUENCE ord_seq;

CREATE OR REPLACE TRIGGER table_res 
BEFORE INSERT ON Orders 
FOR EACH ROW

BEGIN
  SELECT ord_seq.NEXTVAL INTO   :new.Order_ID FROM   dual;
END;
/
   <?php
        $conn=oci_connect("system","123","localhost/orcl");
        ob_start();
        $current_file=$_SERVER['SCRIPT_NAME'];
        $massage= "";
        if(isset($_POST['Cust_id'])&&
        isset($_POST['Order_date']))           
        {

            $Cust_id= $_POST['Cust_id'];
            $Order_date = $_POST['Order_date'];
            if(!empty($Cust_id)&&!empty($Order_date))
            {

                    $sql = "insert into Orders values('".NULL."','".$Cust_id."','".$Order_date."','".NULL."')";
                    $stid = oci_parse($conn,$sql);
                    $r = @oci_execute($stid);
                    if($r)
                    {
                        echo ' data is inserted...<br>';
                    }
                    else
                    {
                        echo 'data was not inserted...<br>';
                    }

            }
            else
            {
                $massage = "please fill up all the form correctly<br>";
            }
        }

    ?>
    <html>
    <head>
    <title>Create FoodItem Table</title>
    <style>
    body
    {
    background:orange;
    }
    </style>
    <head>
    <body>
    You dont need to fill Order_ID and Bill<br><br>
    <?php echo $massage;?>
    <hr color="green">
    <form action="<?php echo $current_file;?>" method="POST">

        Cust_id:<br> <input type="text" name ="Cust_id" ><br><br>
        Order_date:<br> <input type="text" name="Order_date" ><br><br>

        <input type ="submit" value="Submit Order"><br><br>
        //<a href="EmployeeTableshow.php">Home</a>

    </form>
    </body>
    </html>

insert null values like this

$sql="insert into Orders values(NULL,'".$Cust_id."','".$Order_date."',NULL)";

Inserting a NULL value on your primary key is probably not what you want. Either insert a unique value for your key:

 $sql = "insert into Orders values('". $someUniqueValueThatYouCreated . "','".$Cust_id."','".$Order_date."','".NULL."')";

--or--

alter your table structure with, say, an auto_increment:

 Order_ID int(10) primary key auto_increment,

and then modify your insert:

 $sql = "insert into Orders (Cust_id, Order_date, Bill) values('".$Cust_id."','".$Order_date."','".NULL."')";