[php] Extracting a parameter from a URL in WordPress

I am trying to pass a parameter to a WordPress site using a URL - for instance:

www.fioriapts.com/?ppc=1 will be the URL.

I am intending to write a function in the functions.php file but the mechanics of how to extract a parameter in WordPress is beyond me. I am finding a lot of examples on how to add a parameter to a URL using the function add_query_arg() but have found nothing on how to extract a parameter. Thanks in advance for any help.

Why not just use the WordPress get_query_var() function? WordPress Code Reference

// Test if the query exists at the URL
if ( get_query_var('ppc') ) {

    // If so echo the value
    echo get_query_var('ppc');

}

Since get_query_var can only access query parameters available to WP_Query, in order to access a custom query var like 'ppc', you will also need to register this query variable within your plugin or functions.php by adding an action during initialization:

add_action('init','add_get_val');
function add_get_val() { 
    global $wp; 
    $wp->add_query_var('ppc'); 
}

Or by adding a hook to the query_vars filter:

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

In the call back function, use the $request parameter

$parameters = $request->get_params();
echo $parameters['ppc'];

You can try this function

/**
 * Gets the request parameter.
 *
 * @param      string  $key      The query parameter
 * @param      string  $default  The default value to return if not found
 *
 * @return     string  The request parameter.
 */

function get_request_parameter( $key, $default = '' ) {
    // If not request set
    if ( ! isset( $_REQUEST[ $key ] ) || empty( $_REQUEST[ $key ] ) ) {
        return $default;
    }

    // Set so process it
    return strip_tags( (string) wp_unslash( $_REQUEST[ $key ] ) );
}

Here is what is happening in the function

Here three things are happening.

• First we check if the request key is present or not. If not, then just return a default value.
• If it is set, then we first remove slashes by doing wp_unslash. Read here why it is better than stripslashes_deep.
• Then we sanitize the value by doing a simple strip_tags. If you expect rich text from parameter, then run it through wp_kses or similar functions.

All of this information plus more info on the thinking behind the function can be found on this link https://www.intechgrity.com/correct-way-get-url-parameter-values-wordpress/