$ _POST值的内爆错误

I'm try to use the implode function in PHP.

My array has this form:

Array (
    [color] => Yellow
    [size] => 122
    [country] => Array (
        [0] => Canada
        [1] => Brazil
        )
)

How to implode only the country datas to have something like that :

$countries = Canada,Brazil

I have tried:

$countries = implode(",", $_POST['country']);
echo $countries;

Actually $_POST['country'] contains:

[country] => Array (
  [0] => Canada
  [1] => Brazil
)