Doesn't sound like much of a PHP problem, unless you want to reload the page each time to show the changes (which would probably be simpler than writing a pile of Javascript to go through rewriting the page - though that would be faster for the user). Consider having the values of the dropdown options be the conventional ISO symbols for the currency (GBP, USD, EUR). In the script generating the page keep a table of conversion rates indexed by this symbol. Then
if(isset($_POST['currency']) && in_array($_POST['currency'], array_keys($currency_table)))
$conversion_factor = $currency_table[$_POST['currency']];
else
$conversion_factor = 1; // For GBP.
Two reasons for using currency symbols instead of the conversion rates themselves are: (a) it's easier to find the spot where conversion rates are stored for when it's time to change them (b) people have a tendency to mess around with form fields and currency symbols are easier to identify than conversion rates.
