Small currency exchange function (PHP, stand-alone)

exchange.php

<?php
function exchange($amount,$give,$get) {
  $exchanges=[
    'USD:RUB'=>60,
    'RUB:USD'=>1/60,
  ];

  $in=[$give.':'.$get=>$amount];
  $key=key($in);
  if ($amount>0 && $exchanges[$key]) {
     $out=$exchanges[$key]*$in[$key];
     return $out.' '.explode(':',$key,2)[1];
  } else return false;
}

///MAIN////
if ($out=exchange(2,'USD','RUB')) echo $out;
else echo 'Exchange is not possible!';
?>