Why does __sleep() trigger a missing property warning when using property hooks in PHP?

Below is an explanation of what’s happening and why you see the warning:

What’s Happening

In your example, you’ve declared a property with custom accessors using the PHP property hook syntax:

PHPCopied
public int $bar {
    get => $this->foo['baz'];
    set {
        $this->foo['baz'] = $value;
    }
}

When you call get_object_vars($this) inside your __sleep() method, PHP collects what it thinks are the accessible member variables. However, because of property hooks, the accessor property “bar” is not backed by its own storage—it’s just a virtual interface to the private “foo” property.

Why the Warning

Because __sleep() returns the keys produced by get_object_vars($this), “bar” is included in the list of variables that serialize() is supposed to include. But since “bar” is not an actual member (it only exists as a virtual property provided by the hook), serialize() issues a warning:

serialize(): "bar" returned as member variable from __sleep() but does not exist

Essentially, PHP is confused because it’s told to serialize “bar” even though there isn’t a corresponding property to serialize.

Workarounds

You can avoid this problem in a couple of ways:

1. Customize __sleep()
   Instead of blindly returning array_keys(get_object_vars($this)), explicitly return only the properties that are actually stored (in your case, “foo”):

   PHPCopied
   public function __sleep()
   {
       return ['foo'];
   }

2. Use __serialize() (PHP 7.4+/PHP 8.1+)
   If you are able to update your serialization method, you can use the new __serialize() and __unserialize() methods. They allow you to explicitly define the serialized representation without the pitfalls of the older magic methods:

   PHPCopied
   public function __serialize(): array
   {
       return ['foo' => $this->foo];
   }
   
   public function __unserialize(array $data): void
   {
       $this->foo = $data['foo'];
   }

Conclusion

The warning is triggered because of a quirk (or bug) in PHP where property hooks complicate the way __sleep() gathers serializable properties. The property “bar” isn’t causing trouble on its own—it’s just that the automated collection logic doesn’t know that “bar” is only a virtual property. Adjusting your __sleep() (or better yet, using __serialize()) to explicitly list only the stored properties is the way to resolve this issue.