Is there any possible way to call a external php page in wordpress inside another wordpress page

In my case I have created a page named test.php inside my theme folder. I have a specific html structure in test.php file. I don't include header and footer files here. I hard code them in this file because they are different.I validate user details using $_SESSION in function.php. Now what I want to do is, I want to validate the SESSION. If the SESSION is false I need to load the test.php file, else I will follow Wordpress flow.

You mentioned:

I validate user details using $_SESSION in function.php

So I'm assuming you have the validation part down, and are struggling with the redirection to test.php. You can achieve the redirect by hooking in to the template_redirect action.

Inside your functions.php, add:

add_action( 'template_redirect', function() {
    if (!session_is_validated()) {
        include( get_template_directory() . '/test.php' );
        exit;
    }   
});