Discussion on: Daily Challenge #299 - Time to Grille!

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This is the solution in PHP, but I'm not sure if it's 100% correct since there aren't enough test cases and the test cases provided above don't have the result so they're pretty much useless.
anyway, here's my attempt:

function grille($message, $code)
{
    $letters = '';
    $code = array_filter(str_split(sprintf("%06d", decbin($code))), function($x) {
        return $x == 1;
    });
    $code = array_keys($code);
    foreach(str_split($message) as $index => $letter){
        if(in_array($index, $code)){
            $letters .= $letter;
        }
    }
    return $letters;
}