PHP wordpress upload image from URL to media library directly

      require_once(ABSPATH . 'wp-admin/includes/media.php');
        require_once(ABSPATH . 'wp-admin/includes/file.php');
        require_once(ABSPATH . 'wp-admin/includes/image.php');


        $image_id = media_sideload_image($image_url, 0, null, "id");

Second way for existing images in FTP :

<?php $file = $images_DIR . $sku . '.jpg';
        $attachment = [
            'post_mime_type' => 'image/jpeg',
            'post_title' => $sku,
            'post_content' => '',
            'post_status' => 'inherit'
        ];
        $attach_id = wp_insert_attachment($attachment, $file, $product_id);
        $attach_data = wp_generate_attachment_metadata($attach_id, $file);
        wp_update_attachment_metadata($attach_id, $attach_data);
        set_post_thumbnail($product_id, $attach_id);