[WordPress] Maintain WordPress Query String Variables After a Redirect

00-safe-redirect.php

<?php

/**
 * Redirect to the page from which we came (which should always be the
 * admin page. If the referred isn't set, then we redirect the user to
 * the login page.
 *
 * @access private
 */
private function redirect() {

	// To make the Coding Standards happy, we have to initialize this.
	if ( ! isset( $_POST['_wp_http_referer'] ) ) { // Input var okay.
		$_POST['_wp_http_referer'] = wp_login_url();
	}

	// Sanitize the value of the $_POST collection for the Coding Standards.
	$url = sanitize_text_field(
			wp_unslash( $_POST['_wp_http_referer'] ) // Input var okay.
	);

	// Finally, redirect back to the admin page.
	wp_safe_redirect( urldecode( $url ) );
	exit;

}

01-save-function.php

<?php

/**
 * Determines if the current user has permissions to save and, if so, saves
 * each option.
 */
public function save() {

	// Determine if the user has the ability to save the options.
	if ( ! $this->can_save() ) {

		$this->messenger->add_error_message(
			'You do not have permission for this operation.'
		);

	} else {
		$this->save_options()
	}

	// Redirect back to the administration page.
	$this->redirect();

}

02-add-error-message.php

<?php

/**
 * Add an error message to the error message collection. If the incoming
 * message is an array, then an unordered list of errors will be rendered.
 *
 * @param array  $message The message to add to the collection of errors.
 */
public function add_error_message( $message ) {

	// Add the message to the array of errors and then update the option.
	$this->errors[] = esc_html( $message );
	$this->serialize_errors();

}

03-serialize-errors.php

<?php

/**
 * Saves the collection of error messages to the database.
 *
 * @access private
 */
private function serialize_errors() {
	update_option( 'acme-error-messages', $this->errors );
}