WordPress get_query_var()

To troubleshoot, what variables are being used in the request use following code

global $wp_query;
var_dump($wp_query->query_vars);

I managed to get the get_query_var() function to work. To use the two functions successfully, you need to add the query vars to wordpress's query vars array. Here is a code sample.

function add_query_vars_filter( $vars ){
  $vars[] = "query_var_name";
 return $vars;
}

//Add custom query vars
add_filter( 'query_vars', 'add_query_vars_filter' );

Now you can use get_query_var() and add_query_arg() as follows:

Add the query var and value

add_query_arg( array('query_var_name' => 'value'), old_url );

Get the query var value

$value = get_query_var('query_var_name');

More information and code samples can be found at the Codex: get_query_var and add_query_arg