Another way to get data from unknown object in PHP

Question

I'd like to get lastdata of value "user_id" from Object $posts bellow.

I can easily get them with foreach, but I need them directly. Already tried some ways as $posts->entities and $posts->entities[0], but nothing.

I can only get them via foreach as example:

    foreach ($posts as $post) {
    print_r($post);
}

But it would be good if I understand how to get directly.

ArrayCollection {#424 ▼
  #entities: array:2 [▼
    9 => Post {#348 ▼
      #_getterCache: []
      #_valueCache: array:1 [▶]
      #_structure: Structure {#359 ▶}
      #_em: Manager {#211 ▼
        #db: Adapter {#203 ▶}
        #valueFormatter: ValueFormatter {#212}
        #extension: Extension {#205 ▶}
        #entityClassNameMap: array:21 [▶]
        #entities: array:13 [▼
          "XF\Entity\UserPrivacy" => array:2 [▶]
          "XF\Entity\UserProfile" => array:2 [▶]
          "XF\Entity\UserOption" => array:2 [▶]
          "XF\Entity\User" => array:2 [▼
            2 => User {#300 ▼
              #_getterCache: array:1 [▶]
              #_valueCache: array:2 [▶]
              #_structure: Structure {#213 ▶}
              #_em: Manager {#211}
              -_uniqueEntityId: 5
              #rootClass: "XF\Entity\User"
              #_useReplaceInto: false
              #_newValues: []
              #_values: array:34 [▼
                "user_id" => 2
                "username" => "testuser"