Structure of maintainable websites - PHP

Hello,

I'm currently maintaining a website (I developed myself) for the
Intranet of a public school. Among the usual things (news, login system,
online grading) the web interface is also used for controlling the
server it resides on (which includes a few simple tasks such as
verifying the UPS load, adding users, editing a few conf files) and also
it is used for (actually, the main purpose of it)
enabling/restricting/disabling internet access to each of our computer labs.

The system started quite small (since it had to be built fast), but is
starting to grow to incorporate new features. Currently, each function
of the system is built in a different php file. It doesn't use any
template system, though all shared functions are stored in several files
in a /lib directory and imported in an as-needed basis. Some of the
libraries are third party, so some are strictly funcional, others are
Object-Oriented.

I have worked with a few CMSs in the past, indeed I even wrote plugins
for some, so I have *some* idea of application design in PHP. However,
I'd like a few pointers on how to make a good, solid infrastructure for
my code (possibly a complete redesign) which allows it to be easily
maintained by future ****ists. Would using Smarty be the best solution,
in this case? Personally, I wouldn't like to tie the system with any
existing CMS.

So, my question is: are there any good practices that I should be aware
of or example websites I can study, that will give me the knowledge I
need? I'm not looking for references on general software design
approaches, but practical information on how to apply them to PHP websites.

Thanks,
Matheus Degiovani.