AWS Elastic Beanstalk file upload not working

We are using AWS Elastic Beanstalk to host PHP applications that include file uploads that don't work. We have php.ini set tmp_upload_dir to / tmp, but it still doesn't work.

We just moved the site from another server, everything worked fine there, but EB doesn't seem to want to upload files.

Here's some sample code we're using:

$imagePath = "/tmp/";

$allowedExts = array("gif", "jpeg", "jpg", "png", "GIF", "JPEG", "JPG", "PNG");
$temp = explode(".", $_FILES["img"]["name"]);
$extension = end($temp);

if ( in_array($extension, $allowedExts))
  {
  if ($_FILES["img"]["error"] > 0)
    {
         $response = array(
            "status" => 'error',
            "message" => 'ERROR Return Code: '. $_FILES["img"]["error"],
        );
        echo "Return Code: " . $_FILES["img"]["error"] . "<br>";
    }
  else
    {

      $filename = $_FILES["img"]["tmp_name"];
      list($width, $height) = getimagesize( $filename );

      move_uploaded_file($filename, $imagePath . $_FILES["img"]["name"]);

      $response = array(
        "status" => 'success',
        "url" => $imagePath.$_FILES["img"]["name"],
        "width" => $width,
        "height" => $height
      );

    }
  }
else
  {
   $response = array(
        "status" => 'error',
        "message" => 'something went wrong',
    );
  }