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.

$newObj = 开发者_StackOverflow社区(object)$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"
}

The array key contains a marker that this should be a private property of the class test.

Compare your scripts output with the following:

$array = array(
    "xpublic" => "x1", 
    # this will become a private member:
    "\x00test\x00xprivate" => "x2",
    # this will become a protected member:
    "\x00*\x00xprotected" => "x3"
);

var_dump($array);

$obj = (object) $array;

var_dump($obj);

When serialized, the same string is used to describe the private members.

Output:

array(3) {
  ["xpublic"]=>
  string(2) "x1"
  ["testxprivate"]=>
  string(2) "x2"
  ["*xprotected"]=>
  string(2) "x3"
}

object(stdClass)#1 (3) {
  ["xpublic"]=>
  string(2) "x1"
  ["xprivate":"test":private]=>
  string(2) "x2"
  ["xprotected":protected]=>
  string(2) "x3"
}

In the output of var_dump(), the null bytes are not visible.

(Update: Added protected class member)

Probably the PHP engine conserves the class structure internaly and simply gives some kind of an array wrapper, and thus when you cast it again it remains private, though I can't assure this at 100%.