problemer med billed upload (php)

Hej alle sammen, jeg prøver at uploade et profil billed men der er lidt problemer under uploaden.

Den uploader ikke filen :

 $img_name = $_FILES['imga']['name'];
     $img_type = $_FILES['imga']['type'];
     $img_size = $_FILES['imga']['size'];
     $tmpname = $_FILES['imga']['tmp_name'];
     $img_ext = substr($img_name, strrpos($img_name, '.'));
 
         if ($username) {
          move_uploaded_file($tmpname, "avatar/$img_name.$img_ext");                                                        
          $avatar = "$username.$img_ext";
         }