How to used order_id as invoice number and converting any variable as gloabal

I tried multiple times but have been stucked for long time, kindly mention that which method is better S_SESSION or GET Method to used some variable as global. And how to

$date = date('Y-m-d H:i:s');

convert this into SESSION[] ,or as global to access on other pages. Other thing is that how to acces

$i

which is used as order_id , i wanna to used this as my invoice number. How can it is possible The code is give below

<?php include ('header.php'); 
if(isset($_POST['order']))
{
    $user_id  = $_SESSION['id'];
     $date     = date('Y-m-d H:i:s');
    $username =$_SESSION['username'];
    mysql_query("INSERT INTO tborder(user_id,`date`,username) VALUES('$user_id','$date','$username')") or die(mysql_error());
    $i = mysql_insert_id();
    if($i >0)
        for($l=0;$l<count($_POST['product_id']);$l++)
        {
            $product_id = $_POST['product_id'][$l];
            $quantity   = $_POST['quantity'][$l];
            $price      = $_POST['price'][$l];
            $discount   = $_POST['discount'][$l];
            $amount     = $_POST['amount'][$l];
            $username   = $_SESSION['username'];
            $user_id    = $_SESSION['id'];

            mysql_query("INSERT INTO tborderdetail(order_id,product_id,quantity,price,discount,amount,username,user_id) VALUES('$i','$product_id','$quantity','$price','$discount','$amount','$username','$user_id')")or die(mysql_error());
            header('location:order.php');
        }
}
?>
<form action="" method="post">

<div class="row">
    <div class="col-md-6">

  <div class="panel panel-default">
    <div id="Tabs" role="tabpanel">
    <!--- Nav Tabs -->

        <ul class="nav nav-tabs" role="tablist">

        <?php
            $q =mysql_query("SELECT * FROM tbcategory");
            while($row =mysql_fetch_object($q))
            {
        ?>  
                <li class="">
                <a href="#<?=$row->id?>" aria-controls="personal" role="tab" data-toggle="tab"><?=$row->category_name?></a>
                </li>
        <?php
            }
        ?>
        </ul>
        <!--Tab Panes-->
        <div class="tab-content" style="padding-top: 20px">
        <?php
            $q=mysql_query("SELECT * FROM tbcategory");
            while($row = mysql_fetch_object($q))
                {
        ?>      
            <div role="tabpanel" class="tab-pane" id="<?=$row->id?>">
                <?php
                $p =mysql_query("SELECT * FROM tbproduct WHERE cat_id = '$row->id'");
                    while($r = mysql_fetch_object($p))
                    {
                    ?>
                    <div class="col-md-4">
                    <a class="datavalue" href="#" data-id="<?=$r->id?>" data-name="<?=$r->product_name?>" data-price="<?=$r->unit_price ?>">
                        <img width="100" height="100" src="product/<?=$r->image?>">
                        <h5><?=$r->product_name?></h5>
                    </a>
                    </div>
                    <?php
                }
                ?>
            </div>
        <?php 
                }
        ?>  
            </div>
        </div>
        </div>


</div>

    <div class="col-md-6">
    <table class="table">
<center><h2>MY PRODUCT</h2></center>

    <tr>
            <?php
               date_default_timezone_set('Asia/Karachi');
               $currenttime = date('h:i A');
            ?>
        <td>
            <label>Inv Date : </label>
            <?= date('d-M-Y'); ?> 
        </td>
        <td>

            <label>Inv Time : </label>
            <?=$currenttime ?> 
        </td>
    </tr>
    <tr>
        <td>
            <label>Inovice No.</label>
                   ????
        </td>
        <td>
            <label>User Name</label>
            <?=$_SESSION['username'] ?>
        </td>
    </tr>
</table>
        <table class="table table-bordered table-hover">
        <thead>
            <tr>
                <th>ProductName</th>
                <th>Quantity</th>
                <th> Price </th>
                <th>Discount</th>
                <th>Amount</th>
                <th>X</th>
            </tr>
        </thead>
        <tbody class="details">
        </tbody>
        <tfoot>
            <tr>
                <td>
                    SubTotal : <input type="text" name="subtotal" class="form-control subtotal">
                    Cash Receive: <input type="text" name="pay" class="form-control pay">
                    Pay Back : <input type="text" name="payback" class="form-control payback">
                </td>
            </tr>
        </tfoot>    
        </table>
        <input type="submit" name="order" value="Order" class="btn btn-primary">

    </div>
</div>
</form>


<?php include ('footer.php') ;?>