Dynamic Rule Names and Values

OK, I have been mulling this one over for quite a while now. I'll get straight to it.

Here's my rule in the cart file...

//eCart Rule
function WAEC_theCart_TotalUnitThresholdDiscount() {
$totalDiscount = 0;
if (true && (($this->InCart("".(isset($_SESSION['item_3128'])) ? $_SESSION['item_3128'] : '' ."") == true))) {
$totalDiscount += floatVal("".(isset($_SESSION['totaldiscountfor_3128'])) ? $_SESSION['totaldiscountfor_3128'] : '' ."");//Result
}
return WA_eCart_FormatNumber($totalDiscount, $this->ForceDecimalsC, $this->DecimalPlacesC);
}
//eCart Rule

You'll notice in there that there are TWO SESSIONS where their names end '_3128'

Now, my question is this...

How do (or can!?) I make "_3128" dynamic in this rule so that it looks something more like this $_SESSION['totaldiscountfor_DYNAMIC']

That which I wish to do is make the _DYNAMIC part reference the actual ID of the product being passed to the cart, for example, I would love to be able to do this $_SESSION['totaldiscountfor_.$this->ID)']

Would (should) that work, I did try that but it will not...is there a way to do this?

Regards
Andrew

PS: Please assume I have done all the other configurations, including setting the value of said variable...which I definitely have done :-)