Warning: require_once(): http:// wrapper is disabled in the server configuration by allow_url_include=0

The warning is generated because you are using a full URL for the file that you are including. This is NOT the right way because this way you are going to get some HTML from the webserver. Use:

require_once('../web/a.php');

so that webserver could EXECUTE the script and deliver its output, instead of just serving up the source code (your current case which leads to the warning).

I had this same error while trying to include a PHP file in my Wordpress theme. I was able to get around it by referencing the file name using dirname(__FILE__). I couldn't use relative paths since my file was going to be included in different places throughout the theme, so something like require_once '../path-to/my-file' wouldn't work.

Replacing require_once get_template_directory_uri() . '/path-to/my-file' with require_once dirname( __FILE__ ) . '/path-to/my-file' did the trick.

try to use

<?php require_once($_SERVER['DOCUMENT_ROOT'].'/web/a.php'); ?>