Don't store variable if not set

function append_URL( $link, $sort ) {
  $sort = $_GET['sort'];

  if ( isset($sort) ) {
      $link = add_query_arg( 'sort', $sort, $link );
  }
  return $link;   
}

I get this notice Undefined index: sort when no sort parameter exists. What is the best way to check if the sort param exists before creating the $开发者_如何转开发sort variable?

$sort = array_key_exists('sort', $_GET) ? $_GET['sort'] : null;

Also, if you're just setting the $sort value from the $_GET, why do you pass it in as an argument to your function? It seems redundant.

if (isset($_GET['sort']))