Array to Object and Object to Array in PHP - interesting behaviour

Can you explain the next interesting behaviour?

class test {
  //Class *test* has two properties, public and private.
  public $xpublic = 'x1';
  private $xprivate = 'x2';
}
$testObj = new test();

Let's convert $testObj to array.

settype($testObj, 'array');
var_dump($testObj);

Result:

array(2) {
  ["xpublic"]=> string(3) "x1"
  ["testxprivate"]=> string(4) "x2"
}

OK, xprivate property becomes testxprivate

Let's convert this array to object.

$newObj = (object)$testObj;
var_dump($newObj);

Result:

object(stdClass)#1 (2) {
  ["xpublic"]=> string(3) "xxx"
  ["xprivate":"test":private]=> string(4) "xxx3"
}

$newObj is a stdClass object.

And the question is:

Why does testxprivate become a private property xprivate (not testxprivate) of the new object? How does PHP know that $testObj array was an object?

If I define the equal array:

$testArray = array('xpublic'=>'x1', 'testxprivate'=>'x2');

and then convert it to object:

var_dump((object)$testArray);

I'll get the object with two public properties xpublic and testxprivate as expected:

object(stdClass)#2 (2) {
  ["xpublic"]=> string(2) "x1"
  ["testxprivate"]=> string(2) "x2"
}