include header and sidbar into a standalone php file

You don’t need to know “where” those functions are in order to be able to use them on a non-WP file.
You need to make that non_WP file ‘WP-aware’ 🙂
<sigh>it has been posted about 173 times in this forum…</sigh>

Add on the top before everything)
<?php
require('./path-to-your-blog/wp-blog-header.php');
?>

After that any WP function should work on that file.