Multidimensional Array Access

Hello, I'm really stumped as to why the piece of code that I have isn't working, and is in fact giving me an error. I have a multidimensional array ($results) that contains the following:

array(50) {
  [0]=>
  object(stdClass)#9 (18) {
    ["created_at"]=>
    string(31) "Tue, 11 Dec 2012 04:09:29 +0000"
    ["from_user"]=>
    string(9) "jordklotz"
    ["from_user_id"]=>
    int(67114977)
    ["from_user_id_str"]=>
    string(8) "67114977"
    ["from_user_name"]=>
    string(12) "Jordan Klotz"
    ["geo"]=>
    NULL
    ["id"]=>
    float(2.7835077069964E+17)
    ["id_str"]=>
    string(18) "278350770699644928"
    ["iso_language_code"]=>
    string(2) "en"
    ["metadata"]=>
    object(stdClass)#10 (1) {
      ["result_type"]=>
      string(6) "recent"
    }
    ["profile_image_url"]=>
    string(90) "http://a0.twimg.com/profile_images/2763602606/cb1c224c304edf6ce23776a6c81e37df_normal.jpeg"
    ["profile_image_url_https"]=>
    string(92) "https://si0.twimg.com/profile_images/2763602606/cb1c224c304edf6ce23776a6c81e37df_normal.jpeg"
    ["source"]=>
    string(89) "<a href="http://twitter.com/download/iphone">Twitter for iPhone</a>"
    ["text"]=>
    string(29) "RT @tMaCkTiMeS: don't test me"
    ["to_user"]=>
    NULL
    ["to_user_id"]=>
    int(0)
    ["to_user_id_str"]=>
    string(1) "0"
    ["to_user_name"]=>
    NULL
  }
  [1]=>
  object(stdClass)#11 (18) {
    ["created_at"]=>
    string(31) "Tue, 11 Dec 2012 04:09:29 +0000"
    ["from_user"]=>
    string(10) "marianaaem"
    ["from_user_id"]=>
    int(177839089)
    ["from_user_id_str"]=>
    string(9) "177839089"
    ["from_user_name"]=>
    string(18) "Mariana Escalante "
    ["geo"]=>
    NULL
    ["id"]=>
    float(2.7835076986079E+17)
    ["id_str"]=>
    string(18) "278350769860788224"
    ["iso_language_code"]=>
    string(2) "en"
    ["metadata"]=>
    object(stdClass)#12 (1) {
      ["result_type"]=>
      string(6) "recent"
    }
    ["profile_image_url"]=>
    string(89) "http://a0.twimg.com/profile_images/2675105716/6fc399aa6815b67da1744de4057dd2f6_normal.png"
    ["profile_image_url_https"]=>
    string(91) "https://si0.twimg.com/profile_images/2675105716/6fc399aa6815b67da1744de4057dd2f6_normal.png"
    ["source"]=>
    string(59) "<a href="http://twitter.com/">web</a>"
    ["text"]=>
    string(65) "@Harry_Styles @NiallOfficial BOTH ONLINE AND I HAVE TEST TOMORROW"
    ["to_user"]=>
    string(12) "Harry_Styles"
    ["to_user_id"]=>
    int(181561712)
    ["to_user_id_str"]=>
    string(9) "181561712"
    ["to_user_name"]=>
    string(12) "Harry Styles"
  }

You get the gist of it... Now what I want to do is take the "id_str" column of each array within the array, and assign it to a new array. I have tried a couple of different ways, none of which have worked.

Here are a few of my tries...

$tweetIds = array(50);

    // loop through $results and store them
    for($i=0; $i<3; $i++)
    {
        $tweetIds[$i] = $results[$i]["id_str"];
    }

I also tried it with the number 7 in the second access of $results since I know that "id_str" is always in the 7th spot, but that didn't work either.

Also, the 3 in the loop is just for testing purposes.