Casting PHP classes by modifing serialized data

<pre><?php

class A {
    private $a = 'trallala';
    protected $b = array(1,2,3);
    public $foo = 'bar';
}

class B extends A {
    
}

function typecast($v, $toType) {
    
    $fromType = get_class($v);
    $data = serialize($v);
    
    $data = str_replace(
        'O:' . strlen($fromType) . ':"' . $fromType . '":',
        'O:' . strlen($toType) . ':"' . $toType . '":',
        $data
    );
    $data = str_replace("\0$fromType\0", "\0$toType\0", $data);
    return @unserialize($data);
}
$b = typecast(new A(), 'B');