@jezspain

Simply change
if (!isset($_SESSION['currency']) && !isset($_GET['currency']) ) $_SESSION['currency'] = DEFAULT_CURRENCY;
to
if (!isset($_SESSION['currency']) && !isset($_GET['currency']) ) $_SESSION['currency'] = 'EUR';
Then open up a new browser window as the change will work only when there's not an old php session lurking, and you'll see it as you customers will.

This assumes, of course, that you have dollars and euros enabled as currencies, though there is protection in that file for if you don't.