Storing key values in an array

I am using an array to populate a drop-down and I would like to have the same value in the key as the value. Using the following, the key is 0,1,2,3...etc. How can I correct that?

    $file_list = array();
    $dir = opendir($doc_dir);
    while (false !== ($file = readdir($dir))) {
      if (($file != ".") && ($file != ".."))
        $file_list[$file] = $file;  <---------------<<<<<
    }

Todd