PHP: __PHP_Incomplete_Class - Manual

<?php

class MyClass
{
    public string $property = "myValue";
}

$myObject = new MyClass;

$foo = serialize($myObject);

// unserializes all objects into __PHP_Incomplete_Class objects
$disallowed = unserialize($foo, ["allowed_classes" => false]);

var_dump($disallowed);

// unserializes all objects into __PHP_Incomplete_Class objects except those of MyClass2 and MyClass3
$disallowed2 = unserialize($foo, ["allowed_classes" => ["MyClass2", "MyClass3"]]);

var_dump($disallowed2);

// unserializes undefined class into __PHP_Incomplete_Class object
$undefinedClass = unserialize('O:16:"MyUndefinedClass":0:{}');

var_dump($undefinedClass);

object(__PHP_Incomplete_Class)#2 (2) {
  ["__PHP_Incomplete_Class_Name"]=>
  string(7) "MyClass"
  ["property"]=>
  string(7) "myValue"
}
object(__PHP_Incomplete_Class)#3 (2) {
  ["__PHP_Incomplete_Class_Name"]=>
  string(7) "MyClass"
  ["property"]=>
  string(7) "myValue"
}
object(__PHP_Incomplete_Class)#4 (1) {
  ["__PHP_Incomplete_Class_Name"]=>
  string(16) "MyUndefinedClass"
}