PHP: __toString() and json_encode() not playing well together

class A
{
    protected $a;

    public function __construct()
    {
    	$this->a = array( new B, new B );
    }

    public function __toString()
    {
    	return json_encode( $this->a );
    }
}

class B
{
    protected $b = array( 'foo' => 'bar' );

    public function __toString()
    {
    	return json_encode( $this->b );
    }
}

$a = new A();

echo $a;

[{},{}]

[{"foo":"bar"},{"foo":"bar"}]

class a
{
    public function __set( $prop, $value )
    {
    	echo __METHOD__, PHP_EOL;
    	$this->$prop = $value;
    }

    public function __toString()
    {
    	return json_encode( $this );
    }
}

$a = new a;
$a->foo = 'bar';
$a->foo = 'baz';

echo $a;

class a
{
    public $foo;

    public function setFoo( $value )
    {
    	$this->foo = $value;
    }

    public function __toString()
    {
    	return json_encode( $this );
    }
}

$a = new a;
$a->setFoo( 'bar' );

echo $a;

<?php

class a implements IteratorAggregate
{
    public $foo = 'bar';
    protected $bar = 'baz';

    public function getIterator()
    {
    	echo __METHOD__;
    }
}

echo json_encode( new a );

[{"b":{"foo":"bar"}},{"b":{"foo":"bar"}}]

[{"foo":"bar"},{"foo":"bar"}]

class B
{
    protected $b = array( 'foo' => 'bar' );

    public function __get($name)
    {
    	return json_encode( $this->$name );
    }
}

class A
{
    protected $a;

    public function __construct()
    {
        $b1 = new B;
        $b2 = new B;
        $this->a = array( json_decode($b1->b), json_decode($b2->b) );
    }

    public function __toString()
    {
        return json_encode( $this->a );
    }
}

class A
{
    protected $a;

    public function __construct()
    {
        $this->a = array( (string)new B, (string)new B );
    }

    public function __toString()
    {
        return json_encode( $this->a );
    }
}

$this->a = array( json_decode((string)new B), json_decode((string)new B) );

$b1 = new B;
$b2 = new B;
$this->a = array( $b1->toArray(), $b2->toArray() );