Here is one simple PHP Script which gets exchange rate between GBP and USD
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Google currency rates are not accurate google itself says ==> Google cannot guarantee the accuracy of the exchange rates used by the calculator. You should confirm current rates before making any transactions that could be affected by changes in the exchange rates. Foreign currency rates provided by Citibank N.A. are displayed under licence. Rates are for information purposes only and are subject to change without notice. Rates for actual transactions may vary and Citibank is not offering to enter into any transaction at any rate displayed.