get keys of multi level array

I am wondering what I am doing wrong in the following code, because I get error:
Warning: Illegal offset type in isset or empty in D:\xampp\htdocs\testing\4.php on line 5

$_POST[firstkey][secondkey]=12;
if (isset($_POST)) {
  foreach($_POST as $key){
    foreach ($_POST[$key] as $key2 => $value){
    echo "<br />Key=".$key2."value=".$value;
    }
  }
}

PHP:

How would be the correct was echo in the same statement 1st and 2nd key and value?

Thank you

 

I think you want something like this:

$_POST[firstkey][secondkey]=12;
if (isset($_POST)) {
  foreach($_POST as $key=>$value){
    if (is_array($value)) {
       foreach ($value as $key2 => $value2){
          echo "<br />First Key= ".$key. "Second Key=".$key2." value=".$value2;
      }
    }
    else {
      echo "<br />Key=".$key." value=".$value;
      }
    }
}

PHP: