For HTML form array, You need to set tag name to be array in HTML as shown in below example:

<FORM ...>
#The inputs should be inside the loop in the php program which will generate the HTML form.

<input type='hidden' name='product_id[$i]' value='3'>
<input type='text' size=4 name='cost[$i]' value='400'>
<input type='hidden' name='user_id[$i] value='7'>
……
#don’t forget to have $i++
</FORM>


In the php program, you need to use a loop and POST array to get all the values from HTML form. You can refer to below codes:

for($i=0;$i<count($_POST["product_id"]);$i++)  {
  $product_id[$i]=$_POST["product_id"][$i];
  $product_name[$i]=$_POST["product_name"][$i];
  $cost[$i]=$_POST["cost"][$i];
  ...
}


You can refer to below examples about how to send multiple product records on HTML form, retrieve them in PHP program and update the records on mysql. 

http://www.sourcecodester.com/php/3863/updating-multiple-rows-mysql-using-php.html

## http://www.phpeasystep.com/mysql/10.html  --> not avaialble sometimes.

The key is to have array type on input name (like address[$i], $i is the loop index when you display all the products) as shown below
input type='text' size='40' name='address[$i]'
<input type='hidden' name='id[$i]' value='...'>

and retrieve the variable name and value as shown below:
$address= $_POST['address'][$i];

Below statement shows how to get the number of records sent from HTML FORM in PHP program.
$size = count($_POST['address']);