This is my first post on this forum so please excuse any errors I make

I am creating my first ecommerce site and I want to use include files to structure my site.
However I can not get my menu include files to show.
Not sure what I am doing wrong.
Could someone please point out to me the errors of my ways?
I am calling the include file like this: <?php include("main_nav.php") ;?>
The include file is in the same directory and I am also using css to style the site.
Thanks in advance.

