ser_php_T156364.php

<?php
# Acknowledging Stas Malyshev
# https://phabricator.wikimedia.org/T156364#2977719

# Given two classes with the same property name but different visibility
class WithPublic {
	public $property;
	function __construct( $p ) { $this->property = $p; }
	function getProperty() { print $this->property; }
}
class WithPrivate {
	private $property;
	function __construct( $p ) { $this->property = $p; }
	function getProperty() { print $this->property; }
}

$pub  = new WithPublic( 'value' );

# Lets pretend it is held in a permanent cache:
$cache = serialize( $pub );

# Later on NEW code is deployed which change the property signature to private.
# Simulate the class changed:
$mut_pub_to_priv = unserialize(
	str_replace( '10:"WithPublic"', '11:"WithPrivate"', $cache ) );

var_dump( $mut_pub_to_priv );
# class WithPrivate#2 (2) {
#   private $property =>
#   NULL
#   public $property =>
#   string(5) "value"
# }

var_export( $mut_pub_to_priv->getProperty() );
# NULL

# The object restored from cache is not quite the one we expected and cause
# some havoc ( https://phabricator.wikimedia.org/T156364 ). Then Zend and HHVM
# consistently yield "NULL".

# The other way around. Cache with private property restore to class with
# public property


$priv = new WithPrivate( 'value' );
$cache = serialize( $priv );

$mut_priv_to_pub = unserialize(
	str_replace( '11:"WithPrivate"', '10:"WithPublic"', $cache) );

var_dump( $mut_priv_to_pub );
# class WithPublic#4 (2) {
#   public $property =>
#   NULL
#   private $property =>
#   string(5) "value"
# }

var_export( $mut_priv_to_pub->getProperty() );
# On Zend PHP: NULL
# On HHVM 3.15.4, 3.17.1: valueNULL

Eval is https://3v4l.org/TZk8B

Another similar case is https://3v4l.org/QKTGW

Ref: https://phabricator.wikimedia.org/T156364#2977719