Description:
------------
I built a class with a __toString() member function. When I pass an instance of this class to bindParam(), the object is overwritten with the result of __toString().

The result from the code sample suggests that bindParam is overwriting the value of the object with the object's __toString() value.

Reproduce code:
---------------
<?php
class test
{
	private $a = 1;
	
	public function __toString()
	{
		return strval($this->a);
	}
}

$db = new PDO('mysql:host=localhost;dbname=test', 'root', '', array(PDO::ATTR_PERSISTENT => true));
$obj = new test();

$prepared = $db->prepare('SELECT * FROM table1 WHERE 1 = :test');

var_dump($obj);
$prepared->bindParam('test', $obj);
var_dump($obj);
?>

Expected result:
----------------
object(test)#2 (1) {
  ["a:private"]=>
  int(1)
}
object(test)#2 (1) {
  ["a:private"]=>
  int(1)
}

Actual result:
--------------
object(test)#2 (1) {
  ["a:private"]=>
  int(1)
}
string(1) "1"