Always prefer a greater value using IF Statements

Hello,

I'm after a bit of help with a script I have written, I'm pretty new to PHP and struggling to get my head around it.

I have a website which uses a shipping calculator, featuring the dimensions (width, length, height) and the weight (in kg) of a parcel.
I'm using 2 variables; $volumetric and $weight.
The $volumetric variable is calculated as such, (weight * length * height) / 5000.

Here's what the script is intended to do:-
1. Get values from an HTML form. In this case, 'volumetric' and 'weight'.
2. Compare the values to see which is the largest.
3. Return/redirect to the corresponding URL of the product from the web shop.

At the moment it always takes preference to the volumetric value and does not take the weight into consideration.

An example would be as follows:
Customer enters their dimensions of their box size, 10cm x 10cm x 10cm. This works out to be a volumetric weight value of 0.2KG.
If the customer enters the weight for their parcel as say 5KG, it should return the product for 5KG.

Essentially what I'm trying to do is return/redirect to the URL of a product which has the greater value. I'm currently using IF statements to do this, however, I'm always open to suggestions!

I have attached the code for your reference.

Many Thanks in advance for your reply.

Ben M. fetchProduct.php fetchProduct.php