WordPress shortcode to parse URL query string parameters. Drop into functions.php and add to a custom plugin.

ahd-parse-query-string-wp-shortcode.php

<?php
/**
 * Parse query string parameters.
 * @example [ahd_parse_query_string param="parameter_name"]
 */
function ahd_parse_query_string_func( $atts ) {

	// get shortcode attributes
	$atts = shortcode_atts( array(
		'param' => '',
	), $atts );

	$param = $atts[ 'param' ];

	if ( isset( $_GET[$param] ) ) {
		$value = $_GET[$param];
		return htmlspecialchars( $value );
	} else {
		return;
	}
}
add_shortcode( 'ahd_parse_query_string', 'ahd_parse_query_string_func' );