how is mvc architecture used in php without any framework?
It's not. Core PHP is a "start in global namespace statement and expression oriented language". You need extra code (and an optional URL Rewriter) to implement any kind of MVC architecture. That extra code is your framework.
To achieve a MVC pattern you just have to separate your data persistence code ("model", mostly database stuff), the main application logic ("controller") and your presentation to the outside world ("view", like HTML pages or RSS feeds).
IF you just don't mix these three parts in your code, you already have a really basic MVC architecture. Just build distinct classes for your model, view, and controller layers, come up with a well structured way how they talk to each other and then stick to it!
For the sake of code maintainability you should ALWAYS try to work that way.
By writing your own MVC framework that fallows MVC pattern and OOP principles :)
You need to have Front Controller so every HTTP Request goes through one file, index.php, app.php or what ever you want. This way you can configure application in one place.
From there you need Routing mechanism that will analyze HTTP Request, current URL, HTTP Header verb / method, and based on that you will invoke appropriate Controller Method / Action Controller.
From Controller, you can access your Models that will deal with "heavy lifting", deal with database and domain / business logic etc. And from Controller you can render Views.
So you need at least Front Controller, Router / Dispatcher, Controller, Models and Views to have simple MVC arhitecture.
You would do that joust as other MVC web frameworks do, with slight variations depending on your preferences.
Take a look at some simple frameworks like Codeigniter, and read their source code to get idea how they are doing MVC.
And have fun building your MVC! Its all about the fun after all :D
This is how you can use MVC in php without any framework
Try integrating Pear DB Layer, Smarty, PHP GACL in your core code to achieve an MVC architecture.
You can check the PHP MVC Tutorial to find out how to use simple MVC pattern from scratch, not in an existing framework.