Download an image from the URL and attach it to a post in WordPress

download-and-attach.php

<?php
require_once(ABSPATH . 'wp-admin/includes/media.php');
require_once(ABSPATH . 'wp-admin/includes/file.php');
require_once(ABSPATH . 'wp-admin/includes/image.php');
if(!function_exists('wp_get_current_user')) {
	include(ABSPATH . "wp-includes/pluggable.php"); 
}

$url = "http://xyz.ltd/filenam.png";//Just a sample
$file = array();
$file['name'] = $url;
$file['tmp_name'] = download_url($url);
media_handle_sideload( $file, $post_id  );