Introduce PHP::PhpObject to handle repeating stdClass with different property sets

[kaorukobo]

Problem

stdClass object may appears twice or more with difference properties as shown in the example below:

echo serialize([(object)["foo" => 1], (object)["bar" => 2]]);
# => 
#   a:2:{i:0;O:8:"stdClass":1:{s:3:"foo";i:1;}i:1;O:8:"stdClass":1:{s:3:"bar";i:2;}}

However, the current implementation creates Struct::stdClass class with only the foo member
when it met the first serialized object.

Then during processing the 2nd object, it tries to instantiate Struct::stdClass with properties set ["bar" => 2],
and this ends up undefined method bar= error.

Solution

I introduced a new PHP::PhpObject class to represent an unserialized object instead of using Struct.
This is a subclass of OpenStruct so that it can accept any property assignments from serialized objects.

Also it can be re-serialized to what it was before unserializing, keeping both the properties order and the object's class name. (stored in PhpObject#_php_classname)