The "getMethods" method of ReflectionClass also returns the private methods of the parent class.

I'd say it is not correct. Properties and methods defined as private are inherited but you have no access to this properties in child class.
If for example during serializing if you ommit private properties of base class - you'll get wrong object.

class Base{
  private $basevar;
  public function baseFunction(){return $this->basevar;}
}
class Derived extends Base{}

if private properties are not inherited - than calling baseFunction will cause null result and this is not correct in class inheritance.

That is why we have a new serializing interface which allows you to call a serializing function in the base class which then can deal with the private properties in that base class.

With this serializing interface (if you mean __sleep and __wakeup method) and with reflection can not work with private variables, I can not write serialization method that can be used in all child classes .In every child class Reflection will have child instance and will not have access for private variables of parent. 

Reproduce code:
---------------
class A
{
	public function __sleep() 
	{
		$refl = new ReflectionObject($this);
		$props = $refl->getProperties();
		$result = array();
		foreach($props as $prop)
			$result[] = $prop->getName();
		return $result ;
	}
	private $privateVar = 'Test Private';
	public $publicVar = 'Test Public';
	
	public function setPublic($value)
	{
		$this->publicVar = $value;
	}
	public function setPrivate($value)
	{
		$this->privateVar = $value;
	}
}

class B extends A{}

$instance = new B();
$instance->setPrivate('Set Test Private');
$instance->setPublic('Set Test Private');

var_dump($instance);
var_dump(unserialize(serialize($instance)));

Expected result:
----------------
Object before serializing and after is same.

Actual result:
--------------
object(B)#1 (2) {
  ["privateVar:private"]=>
  string(16) "Set Test Private"
  ["publicVar"]=>
  string(16) "Set Test Private"
}
object(B)#3 (2) {
  ["privateVar:private"]=>
  string(12) "Test Private"
  ["publicVar"]=>
  string(16) "Set Test Private"
}


--------------
When writing 
public function __sleep(){return array('privateVar', 'publicVar');} In B class
Than you'll get 

Notice: serialize() [function.serialize]: "privateVar" returned as member variable from __sleep() but does not exist.

So you can't write custom serializing for child object when private properties exists in parent class.

Maybe it is not valid to have private variables of parent class in Reflection. Then it is only my own custom serialization problem.

Simple fix:

Index: php_reflection.c
===================================================================
RCS file: /repository/php-src/ext/reflection/php_reflection.c,v
retrieving revision 1.164.2.33.2.45.2.6
diff -u -r1.164.2.33.2.45.2.6 php_reflection.c
--- php_reflection.c    31 Dec 2007 07:17:13 -0000      1.164.2.33.2.45.2.6
+++ php_reflection.c    16 Jan 2008 02:14:41 -0000
@@ -519,7 +519,8 @@
                        zend_hash_internal_pointer_reset_ex(&ce->function_table, &pos);
 
                        while (zend_hash_get_current_data_ex(&ce->function_table, (void **) &mptr, &pos) == SUCCESS) {
-                               if (!(mptr->common.fn_flags & ZEND_ACC_STATIC)) {
+                               if (!((mptr->common.fn_flags & ZEND_ACC_STATIC) || 
+                                       ((mptr->common.fn_flags & ZEND_ACC_PRIVATE) && mptr->common.scope != ce))) {
                                        char *key;
                                        uint key_len;
                                        ulong num_index;

Assigned to the Reflection maintainer.

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.