If you want super-lightweight, you could use an auto-prepend file, mixed with some output buffering to build what you want. For starters, you need to set up your prepend and append files - put the following lines in your .htaccess file (you'll probably want to make the prepend and append files unreadable to visitors, too):
php_value auto_prepend_file prepend.php
php_value auto_append_file append.php
Then in your prepend.php file, you'll want to turn on output buffering:
In append.php, you'll want to grab the contents of the output buffer, clear the buffer and do any other processing of the content you want (in this example, I set a default page title).
<?php echo $page_title; ?>
After this, you can write each page normally. Here's an example index.php
This is the page
...and an example other.php that does something halfway interesting:
This is the page
enjoy some PHP...
And you're done. You can grow on this a bit, such as initializing DB connection in the prepend, but at some point, you'll probably want to move to a more abstract system that breaks out of a fixed mapping of URLs to paths and files.
