Tend towards fewer includes
With an opcode cache you are better off including fewer files that are larger because each include results in a disk-touching stat() system call.

Don't
<?php
  include 'a.inc';
  include 'b.inc';
  include 'c.inc';
  include 'd.inc';
  include 'e.inc';
?>
Do
<?php
  include './abcde.inc';
?>