Convert stdClass object to array in PHP

Question

I fetched post_id from postmeta:

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

when I try to print_r($post_id); I have an array like this:

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

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

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

)

I want to traverse it. Expected array:

Array
(
    [0]  => 140


    [1] => 141


    [2] => 142

)

How can I do this?