- urldecode: documentation ( source)
- htmlspecialchars_decode: documentation ( source)
<?php
$str = 'MerID=I0110DQX&OrderID=591&ResponseCode=1&ReasonCode=1&ReasonCodeDesc=Approved%2C+no+balances+available&PurchaseAmt=97.24&PurchaseCurrency=980&RRN=010156764656&AuthCode=208851&AmountRefunded=000000000000&AmountReversed=000000000000&ActualAmount=000000009724&AcqID=414963&Time=19%2F08%2F2013+10%3A36%3A42&IP=217.117.64.236&Phone=380957700418&OrderDescription=Ticket+purchase+from+http%3A%2F%2Fprivatbank.ua%2F&PaddedCardNo=5XXXXXXXXXXXX5658&fee=000000000000&eci=7&CountryBin=000&CountryIP=000&OnUs=1&CountryPhone=000&Signature=EjLR8GBlAM0dsBnZ46jy7opuEF4%3D';
echo urldecode(htmlspecialchars_decode($str, ENT_QUOTES));