I want to create a similar search function using php/mysql and Dreamweaver as the following page:-

It seems to use the same page for the results rather than paasing variables via a url

www.whatcar.co.uk/car-comparison.aspx

Is this possible/how?

you can use the same page for results by passing data back to the same page, then at the top, in your php, you sort out which way the page has been called - a good example is lists that get resorted when you click on the column headings. Often these are all done by the same page, with different queries being executed depending on what was clicked.
If you pick apart the DW code you'll find that it often calls itself