Take advantage of your cache
A common thing people do is to have some sort of config.ini file that they include on every request. Typically such a file looks like this:

<?php
  $config['install_path'] = "/usr/share/htdocs";
  $config['db_type'] = "mysql";
  $config['db_user'] = "nobody";
  $config['db_pwd'] = "";
?>
APC has apc_store() and apc_fetch() which is perfect for something like this and it is trivial to implement.

<?php
  if(!$config = apc_fetch('config')) {
    include './config.inc';
    apc_store('config',$config);
  }
?>
We break our own rule of avoiding conditional includes, but this is one that will only be done on the very first request after server startup. And you can always update the config without restarting the server by doing an apc_store() on top of it.

The other way you can do this, which doesn't require an opcode cache nor any shared memory is to make use of the --with-config-file-scan-dir mechanism which allows you to specify a directory that will be scanned for ini files. Any foo.ini file in this directory will be read on server startup and you then use get_cfg_var() to access the settings. The one downside to this approach is that you have to restart the server to make any changes to the config.

<?php
// load_list takes a text file and turns it
// into a global array cached by APC.
function load_list($name) {
  global $$name;
  if(!$$name = apc_fetch($name)) {
    $$name = explode("\n",trim(file_get_contents($name.'.txt')));
    apc_store($name,$$name);
  }
}
?>