It’s pretty easy to simply grab a URL Query Parameter, but here’s how to do it safely.
/**
* 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 ] ) );
}
Snippet pulled from Get URL query parameter values in WordPress to use with plugins/themes (intechgrity.com)