If you are looking to use wordpress features (or) wordpress functions in core php sites (or) outside of wordpress now its possible.

By calling wp-load.php file you can access whole wordpress functions.

<?php 
require( '../my-wodpress/wp-load.php' ); 
get_header(); 
echo 'new content outside WordPress'; 
get_footer(); 
?>

The above code will display wordpress header and footer with echo statement.