usort issue with decimal numbers

See usort docs. Float result will be converted to integer. For correct work use this code:

usort(
    $data, 
    function($a, $b) {
        $result = 0;
        if ($a[$_GET['sortby']] > $b[$_GET['sortby']]) {
            $result = 1;
        } else if ($a[$_GET['sortby']] < $b[$_GET['sortby']]) {
            $result = -1;
        }
        return $result; 
    }
);

Try strnatcmp():

usort($output, function($a, $b) {
    return strnatcmp($b->days_left, $a->days_left);
});