A replacement function for WordPress's get_query_var() that addresses the issue of query variables not being set on some servers, by parsing the globals directly.

safe_get_query_var.php

<?php
/**
 * Get the query variable even if the server rewrite rules do not support it.
 * This is a workaround for the issue where the query vars are not set in the globals.
 *
 * @global WP_Query $wp_query WordPress Query object.
 *
 * @param string $query_var     The variable key to retrieve.
 * @param mixed  $default_value Optional. Value to return if the query variable is not set.
 *                              Default empty string.
 * @return mixed Contents of the query variable.
 */
function safe_get_query_var( $query_var, $default_value = '' ) {
	global $wp_query;

	// Check $wp_query->query_vars first
	if ( isset( $wp_query->query_vars[ $query_var ] ) ) {
		return $wp_query->query_vars[ $query_var ];
	}

	// Fallback to $_GET
	if ( isset( $_GET[ $query_var ] ) ) {
		return sanitize_text_field( wp_unslash( $_GET[ $query_var ] ) );
	}

	// Fallback to $_POST
	if ( isset( $_POST[ $query_var ] ) ) {
		return sanitize_text_field( wp_unslash( $_POST[ $query_var ] ) );
	}

	// Parse REQUEST_URI as a last resort
	static $cached = null;
	if ( $cached === null ) {
		$request = isset( $_SERVER['REQUEST_URI'] ) ? sanitize_url( wp_unslash( $_SERVER['REQUEST_URI'] ) ) : '';
		$parts   = explode( '?', $request, 2 );
		$cached  = [];
		if ( isset( $parts[1] ) ) {
			$query_pairs = explode( '&', $parts[1] );
			foreach ( $query_pairs as $pair ) {
				list($key, $value)           = array_pad( explode( '=', $pair, 2 ), 2, null );
				$cached[ urldecode( $key ) ] = urldecode( $value );
			}
		}
	}

	// Return the variable if found in the parsed query string
	if ( isset( $cached[ $query_var ] ) ) {
		return sanitize_text_field( wp_unslash( $cached[ $query_var ] ) );
	}

	// Return the default value if the variable is not found
	return $default_value;
}