PHP :编码嵌套JSON数组问题

function encode_json($data) {
    $json = '';
    if (is_array($data)) {
        $is_assoc = false;
        foreach ($data as $key => $value) {
            if (!is_numeric($key)) {
                $is_assoc = true;
                break;
            }
        }
        $json .= $is_assoc ? '{' : '[';
        foreach ($data as $key => $value) {
            if ($is_assoc) {
                $json .= '"' . $key . '":';
            }
            if (is_array($value)) {
                $json .= encode_json($value);
            } else {
                $json .= json_encode($value);
            }
            $json .= ',';
        }
        $json = rtrim($json, ',');
        $json .= $is_assoc ? '}' : ']';
    }
    return $json;
}

$data = array(
    'name' => 'John',
    'age' => 30,
    'languages' => array('PHP', 'JavaScript', 'Python')
);

$json = encode_json($data);
echo $json;