redirects.php

<?php
add_action( 'muplugins_loaded', function() {
	if ( defined( 'WP_CLI' ) && WP_CLI ) {
		return;
	}
	
	if ( empty( $_SERVER['HTTP_HOST'] ) ) {
		return;
	}
	
	// Redirect to primary domain
	if ( strtolower( $_SERVER['HTTP_HOST'] ) !== 'www.primary.org' ) {
		wp_redirect( 'https://www.primary.org' . $_SERVER['REQUEST_URI'], 301 );
		exit;
	}
} );