convert file to utf8

function file_to_utf8($file) {
  $contents = file_get_contents($file);
  $contents = mb_convert_encoding($contents, 'UTF-8', mb_detect_encoding($contents, 'UTF-8, ISO-8859-1', true));
  file_put_contents($file, $contents);  
}

file_to_utf8("/path/to/file");