To make the round trip "The WordPress Way" on the "front-end" (doesn't work in the context of wp-admin), you need to use 3 WordPress functions:

• add_query_arg() - to create the URL with your new query variable ('c' in your example)
• the query_vars filter - to modify the list of public query variables that WordPress knows about (this only works on the front-end, because the WP Query is not used on the back end - wp-admin - so this will also not be available in admin-ajax)
• get_query_var() - to retrieve the value of your custom query variable passed in your URL.

Note: there's no need to even touch the superglobals ($_GET) if you do it this way.

Example

On the page where you need to create the link / set the query variable:

if it's a link back to this page, just adding the query variable

<a href="<?php echo esc_url( add_query_arg( 'c', $my_value_for_c ) )?>">

if it's a link to some other page

<a href="<?php echo esc_url( add_query_arg( 'c', $my_value_for_c, site_url( '/some_other_page/' ) ) )?>">

In your functions.php, or some plugin file or custom class (front-end only):

function add_custom_query_var( $vars ){  $vars[] = "c";  return $vars;}add_filter( 'query_vars', 'add_custom_query_var' );

On the page / function where you wish to retrieve and work with the query var set in your URL:

$my_c = get_query_var( 'c' );

On the Back End (wp-admin)

On the back end we don't ever run wp(), so the main WP Query does not get run. As a result, there are no query vars and the query_vars hook is not run.

In this case, you'll need to revert to the more standard approach of examining your $_GET superglobal. The best way to do this is probably:

$my_c = filter_input( INPUT_GET, "c", FILTER_SANITIZE_STRING );

though in a pinch you could do the tried and true

$my_c = isset( $_GET['c'] ? $_GET['c'] : "";

or some variant thereof.

There are quite few solutions to tackle this issue. First you can go for a plugin if you want:

• WordPress Quickie: Custom Query String Plugin

Or code manually, check out this post:

• Passing Query String Parameters in WordPress URL

Also check out:

• add_query_arg

Since this is a frequently visited post i thought to post my solution in case it helps anyone. In WordPress along with using query vars you can change permalinks too like this

www.example.com?c=123 to www.example.com/c/123

For this you have to add these lines of code in functions.php or your plugin base file.

From shankhan's anwer

add_filter( 'query_vars', 'addnew_query_vars', 10, 1 );function addnew_query_vars($vars){       $vars[] = 'c'; // c is the name of variable you want to add           return $vars;}

And additionally this snipped to add custom rewriting rules.

function custom_rewrite_basic() {    add_rewrite_rule('^c/([0-9]+)/?', '?c=$1', 'top');}add_action('init', 'custom_rewrite_basic');

For the case where you need to add rewrite rules for a specifc page you can use that page slug to write a rewrite rule for that specific page. Like in the question OP has asked about

www.example.com/news?c=123 to www.example.com/news/123

We can change it to the desired behaviour by adding a little modification to our previous function.

function custom_rewrite_basic() {    add_rewrite_rule('^news/([0-9]+)/?', 'news?c=$1', 'top');}add_action('init', 'custom_rewrite_basic');

Hoping that it becomes useful for someone.