insert an html tag using php plugin

You shouldn’t edit the WP Core because it will break at the next update. I believe the easiest way is to use jQuery. Use a filter to add the jQuery code to the page.

function cystom_jquery_code() {
   echo '<script>
   jQuery("body").after("div class="myClass"></div>");
   jQuery("#wpwrap").before("<div class="beforeClass"></div>");
   </script>';
}

add_action('admin_head', 'cystom_jquery_code');