Well, actually not. The closest thing to overall_header.tpl and overall_footer.tpl would be main.tpl. The following is what happens when a page is displayed in PunBB:
1. header.php is executed. In this script the main.tpl template is loaded into memory. The replacement variables pun_content_direction, pun_char_encoding, pun_head, pun_body, pun_title, pun_desc, pun_navlinks and pun_status are replaced with real content.
2. Next, the script that we are actually calling is executed (i.e. viewtopic.php). This script replaces pun_main in the main template with real content.
3. Finally, footer.php is executed. This script replaces pun_footer with real content. At the end of footer.php the contents of the main template (that we have been "filling out" with content in all three scripts) is outputted.
Thus, if you wanted to add a logo to your forum pages, you would just edit main.tpl and put the HTML code for the image somewhere. If you, however, wanted to run some of your own PHP code, you would have to hack header.php, footer.php or any other script that the code would be suited for.
"Programming is like sex: one mistake and you have to support it for the rest of your life."