我从postmeta获取post_id为：

$post_id = $wpdb->get_results("SELECT post_id FROM $wpdb->postmeta WHERE (meta_key = 'mfn-post-link1' AND meta_value = '". $from ."')");

当我尝试print_r($post_id); 我有这样的数组：

Array
(
    [0] => stdClass Object
        (
            [post_id] => 140
        )

    [1] => stdClass Object
        (
            [post_id] => 141
        )

    [2] => stdClass Object
        (
            [post_id] => 142
        )

)

而且我不知道如何遍历它，如何获得这样的数组

Array
(
    [0]  => 140


    [1] => 141


    [2] => 142

)

知道我该怎么做吗？