Well you never sent that request to the twitter server.. What you did is just created a url string and then just used json_decode that same string. No API function was called so some data can be returned. 
In your browser go to the url string you made. You will get the JSON output.
Either you must use an API function and pass this url to that funciton so that it returns something else use curl to do a request to that url manually and get the data and then json_decode it.
EDIT : With the edited question, I cant reproduce the error. Its working fine here. see this output.
array (size=2)
0 => 
object(stdClass)[2]
  public 'created_at' => string 'Thu Dec 20 11:11:29 +0000 2012' (length=30)
  public 'id' => float 2.8171845961476E+17
  public 'id_str' => string '281718459614760960' (length=18)
  public 'text' => string 'RPF’S 25-YEAR JOURNEY - The New Times Rwanda http://t.co/BvEuZz06 via @NewTimesRwanda' (length=87)
  public 'source' => string '<a href="http://twitter.com/tweetbutton" rel="nofollow">Tweet Button</a>' (length=72)
  public 'truncated' => boolean false
  public 'in_reply_to_status_id' => null
  public 'in_reply_to_status_id_str' => null
  public 'in_reply_to_user_id' => null
  public 'in_reply_to_user_id_str' => null
  public 'in_reply_to_screen_name' => null
  public 'user' => 
    object(stdClass)[3]
      public 'id' => int 17686705
      public 'id_str' => string '17686705' (length=8)
      public 'name' => string 'Angelica Gasaro' (length=15)
      public 'screen_name' => string 'agasaro' (length=7)
      public 'location' => string 'SW florida' (length=10)
      public 'url' => null
      public 'description' => string '' (length=0)
      public 'protected' => boolean false
      public 'followers_count' => int 63
      public 'friends_count' => int 43
      public 'listed_count' => int 0
      public 'created_at' => string 'Thu Nov 27 19:42:59 +0000 2008' (length=30)
      public 'favourites_count' => int 5
      public 'utc_offset' => int -18000
      public 'time_zone' => string 'Eastern Time (US & Canada)' (length=26)
      public 'geo_enabled' => boolean true
      public 'verified' => boolean false
      public 'statuses_count' => int 70
      public 'lang' => string 'en' (length=2)
      public 'contributors_enabled' => boolean false
      public 'is_translator' => boolean false
      public 'profile_background_color' => string 'B2DFDA' (length=6)
      public 'profile_background_image_url' => string 'http://a0.twimg.com/images/themes/theme13/bg.gif' (length=48)
      public 'profile_background_image_url_https' => string 'https://si0.twimg.com/images/themes/theme13/bg.gif' (length=50)
      public 'profile_background_tile' => boolean false
      public 'profile_image_url' => string 'http://a0.twimg.com/profile_images/2849801480/1d045f13f7b8d248516900a1960be54c_normal.jpeg' (length=90)
      public 'profile_image_url_https' => string 'https://si0.twimg.com/profile_images/2849801480/1d045f13f7b8d248516900a1960be54c_normal.jpeg' (length=92)
      public 'profile_banner_url' => string 'https://si0.twimg.com/profile_banners/17686705/1352931714' (length=57)
      public 'profile_link_color' => string '93A644' (length=6)
      public 'profile_sidebar_border_color' => string 'EEEEEE' (length=6)
      public 'profile_sidebar_fill_color' => string 'FFFFFF' (length=6)
      public 'profile_text_color' => string '333333' (length=6)
      public 'profile_use_background_image' => boolean true
      public 'default_profile' => boolean false
      public 'default_profile_image' => boolean false
      public 'following' => null
      public 'follow_request_sent' => null
      public 'notifications' => null
  public 'geo' => null
  public 'coordinates' => null
  public 'place' => null
  public 'contributors' => null
  public 'retweet_count' => int 0
  public 'favorited' => boolean false
  public 'retweeted' => boolean false
  public 'possibly_sensitive' => boolean false
1 => 
object(stdClass)[4]
  public 'created_at' => string 'Sun Dec 09 15:19:29 +0000 2012' (length=30)
  public 'id' => float 2.777946036069E+17
  public 'id_str' => string '277794603606896640' (length=18)
  public 'text' => string '“@RichardDreyfuss: The Island Where People Forget to Die http://t.co/rhN3s5E9â€' (length=82)
  public 'source' => string '<a href="http://twitter.com/download/iphone" rel="nofollow">Twitter for iPhone</a>' (length=82)
  public 'truncated' => boolean false
  public 'in_reply_to_status_id' => null
  public 'in_reply_to_status_id_str' => null
  public 'in_reply_to_user_id' => null
  public 'in_reply_to_user_id_str' => null
  public 'in_reply_to_screen_name' => null
  public 'user' => 
    object(stdClass)[5]
      public 'id' => int 17686705
      public 'id_str' => string '17686705' (length=8)
      public 'name' => string 'Angelica Gasaro' (length=15)
      public 'screen_name' => string 'agasaro' (length=7)
      public 'location' => string 'SW florida' (length=10)
      public 'url' => null
      public 'description' => string '' (length=0)
      public 'protected' => boolean false
      public 'followers_count' => int 63
      public 'friends_count' => int 43
      public 'listed_count' => int 0
      public 'created_at' => string 'Thu Nov 27 19:42:59 +0000 2008' (length=30)
      public 'favourites_count' => int 5
      public 'utc_offset' => int -18000
      public 'time_zone' => string 'Eastern Time (US & Canada)' (length=26)
      public 'geo_enabled' => boolean true
      public 'verified' => boolean false
      public 'statuses_count' => int 70
      public 'lang' => string 'en' (length=2)
      public 'contributors_enabled' => boolean false
      public 'is_translator' => boolean false
      public 'profile_background_color' => string 'B2DFDA' (length=6)
      public 'profile_background_image_url' => string 'http://a0.twimg.com/images/themes/theme13/bg.gif' (length=48)
      public 'profile_background_image_url_https' => string 'https://si0.twimg.com/images/themes/theme13/bg.gif' (length=50)
      public 'profile_background_tile' => boolean false
      public 'profile_image_url' => string 'http://a0.twimg.com/profile_images/2849801480/1d045f13f7b8d248516900a1960be54c_normal.jpeg' (length=90)
      public 'profile_image_url_https' => string 'https://si0.twimg.com/profile_images/2849801480/1d045f13f7b8d248516900a1960be54c_normal.jpeg' (length=92)
      public 'profile_banner_url' => string 'https://si0.twimg.com/profile_banners/17686705/1352931714' (length=57)
      public 'profile_link_color' => string '93A644' (length=6)
      public 'profile_sidebar_border_color' => string 'EEEEEE' (length=6)
      public 'profile_sidebar_fill_color' => string 'FFFFFF' (length=6)
      public 'profile_text_color' => string '333333' (length=6)
      public 'profile_use_background_image' => boolean true
      public 'default_profile' => boolean false
      public 'default_profile_image' => boolean false
      public 'following' => null
      public 'follow_request_sent' => null
      public 'notifications' => null
  public 'geo' => null
  public 'coordinates' => null
  public 'place' => null
  public 'contributors' => null
  public 'retweet_count' => int 0
  public 'favorited' => boolean false
  public 'retweeted' => boolean false
  public 'possibly_sensitive' => boolean false 
NOTE : check the links
file_get_contents throws 400 Bad Request error PHP  &  https://dev.twitter.com/discussions/678
May be you have reached the api limit for the day thats why they are blocking you and sending you a 400 response.