how to invoke wordpress API from other existing PHP system

Question

We have our own php system, now we want to have a blog function. Because wordpress are widely used, so we decided to use it. And now I want to invoke wordpress's function , for example wp_insert_user , how can I can do that ?

Suppose my system is A and installed on /var/www/A, and wordpress is WP and installed on /var/www/WP. I can put them together like this /var/www/A/WP, but I don't think that's a good idea.

If we use /var/www/A and /var/www/WP, how can I invoke wordpress's function from a php file that in project A? Using curl?

Edit:=======================================================

Now I put WP below A (/var/www/A/wordpress)

Here, in my project A , I have a logon.php, what I want to do is invoke 'wp_insert_user' function to create a new wordpress user when user logon our system (just a test purpose).

I put the following code at the beginning of my logon.php

define('WP_USE_THEMES', false);
require('/var/www/A/wordpress/wp-blog-header.php');

Before I add the two line, when I visit my site, the url is :

http://localhost/myproject/www/logon.php

After I add it, the url become this:

http://localhost/myproject/www/logon.php/wp-admin/install.php

what worse, all the style of my logon.php are completely gone.

My requirement is simple:

when user logon our system, invoke wordpress function 'wp_insert_user' 
to create a user for wordpress. Just to create a new user, no more action.

Answer 1

You can add the following snippet at the start of your PHP file to load WordPress in it.

define( 'WP_USE_THEMES', false );
require('wp-blog-header.php'); // chage the path to match your WP directory
//the rest of your code here

EDIT: Alternatively, you could use wp-load.php instead of wp-blog-header.php.

define( 'WP_USE_THEMES', false );
require('wp-load.php'); // chage the path to match your WP directory
//the rest of your code here

This answer explains that:

It loads all of WordPress, but doesn't call wp() or invoke the template loader (used by themes). Method 2 will be a little lighter-weight, but should give you the same functionality.

Answer 2

Like RRikesh said, including wp-blog-header.php will work but it also has lines of code that includes theme templates. You can take it one step further and just include wp-load.php so you have the bare bones, no extra frills or extra stuff to load.