Some people would like to create a page which is external to SMF, and does not use a |Add_a_custom_action_using_integration_hooks, but still displays the theme header and footer.
The following code will display the forum's header and footers, as well as a full forum page initiation (all variables, etc) but will allow you to include your own content in the middle.
<?php require_once('/path/to/SSI.php'); template_html_above(); template_body_above(); echo ' <hr /> This would be the content sections of the page <hr /> '; template_body_below(); template_html_below(); ?>