Most tgps will not accept .php extensions because some nasty things can be done with php.
Structure - they give fuck all about it as long as the page exists.
If using index.html as your extension MAKE SURE you include index.html ie:
http://www.domain.com/directory/gallery/index.html in the url and not just
http://www.domain.com/directory/gallery/ as some servers will report this as a redirect.
Personally, I wouldn't use sequential numbers in the directories... freeloaders will often change directories looking for more... so mix them up, or use letter / number combos and get some 404s. Then take those 404s and feed trades / show FPAs... whatever