This is my code, I hope this is what are you looking for.
First I create a new array $output to make it more easy to search  
$string = file_get_contents("http://www.mycurrency.net/service/rates");
$json = json_decode($string, true);
foreach ($json as $key => $data) {
    $output[$key]['code'] = $data['code'];
    $output[$key]['rate'] = $data['rate'];
}
After that we use a function to search value in array and returning the key. I got it from here
function searchForRate($countryCode, $array) {
   foreach ($array as $key => $val) {
       if ($val['code'] === $countryCode) {
           return $key;
       }
   }
   return null;
}
and then I run the function with the first parameter as country code to get the keys of specific country code.
$find = searchForRate("BT", $output);
And then echo the rates from our $output array by key in $find variable
echo 'RATE = '.$output[$find]['rate'];
This is the complete codes
<?php
$string = file_get_contents("http://www.mycurrency.net/service/rates");
$json = json_decode($string, true);
foreach ($json as $key => $data) {
    $output[$key]['code'] = $data['code'];
    $output[$key]['rate'] = $data['rate'];
}
function searchForRate($countryCode, $array) {
   foreach ($array as $key => $val) {
       if ($val['code'] === $countryCode) {
           return $key;
       }
   }
   return null;
}
$find = searchForRate("BT", $output);
echo 'RATE = '.$output[$find]['rate'];
Example output:
RATE = 64.13