What about this?

class EntryPoint {
  public object $privateCollaborator;

  public function __construct() {
    $this->privateCollaborator = self::getPrivateCollaborator();
  }

  private static function getPrivateCollaborator() {
    return new class {
    };
  }
}

$entryPoint = new EntryPoint();

I sometimes use it so as not to dirty the LSP-Intelephense autocomplete in Sublime Text. If I remove the object type to the property at the entrance point, LSP-Intelephense will recognize the reference of the object, being able to self-fulfill the members of that collaborator