How to Create new Session Variables

I want to make a download centre but cannot create the session variables that I need to do it.

The method I want to use works as follows.

In my products table I have 3 extra fields,

1. DownloadPath
2. UserID
3. DigitalOrPhysical

These fields are used when products are added.

On my download page, I simply make a recordset, which filters the UserID Session variable to ensure the User only gets what they paid for. I then filter the DigitalOrPhysical field which would ensure only Digital products display (As Physical Products would also be stored in the same table).

And finally I would make a download button using the DownloadPath field.

However my problem is that when the order details are stored in the database the user Id is submitted successfully but the DownloadPath, and DigitalOrPhysical fields are not.

What I think I need to do is simply create session variables when the user clicks the button to checkout on the cart page.

However I am not sure as I am not too familiar with PHP.

Thanks in advance