index.php

<?php
 /*
	 * Plugin Name: WooCommerce Search by SKU
	 * Description: Enhance WooCommerce search to include SKU.
	 * Version: 1.0
	 * Requires at least: 5.2
	 * Requires PHP: 7.2
	 * Author: P4UL
	 * Author URI: https://www.p4ul.dev
	 * License: DBAD
	 * License URI: https://dbad-license.org/
	 * Text Domain: woocommerce-search-by-sku
 */

	if(!defined('ABSPATH')):
		exit; // Exit if accessed directly.
	endif;

	function search_by_sku($search, $wp_query) {
		global $wpdb;

		if(is_admin() || !$wp_query->is_search() || !$wp_query->is_main_query()):
			return $search;
		endif;

		// GET THE SEARCH QUERY
		$search_query = $wp_query->get('s');
		if(empty($search_query)):
			return $search;
		endif;

		// CHECK IF WOOCOMMERCE IS ACTIVE
		if(!class_exists('WooCommerce')):
			return $search;
		endif;

		// MODIFY SEARCH TO INCLUDE SKU
		$search = " AND (";
		$search .= " {$wpdb->posts}.post_title LIKE '%".esc_sql($search_query)."%'";
		$search .= " OR {$wpdb->posts}.post_content LIKE '%".esc_sql($search_query)."%'";
		$search .= " OR EXISTS (SELECT * FROM {$wpdb->postmeta} WHERE {$wpdb->postmeta}.meta_key = '_sku' AND {$wpdb->postmeta}.meta_value LIKE '%".esc_sql($search_query)."%' AND {$wpdb->postmeta}.post_id = {$wpdb->posts}.ID)";
		$search .= ")";

		return $search;
	}
	add_filter('posts_search', 'search_by_sku', 10, 2);