phpBB 3.1 requires PHP 5.3.3+, so some code can be simplified.

includes/startup.php

Lines 18 to 21 (code) can be removed.

Lines 123 to 148 (code) can be replaced with:

date_default_timezone_set(@date_default_timezone_get());

All of this because E_DEPRECATED, date_default_timezone_set() and date_default_timezone_get() were introduced in PHP 5.3.0.

includes/functions.php

Line 772 (code) can be replaced with the following for clarity:

// Check if we are able to resolve symlinks, Windows (prior to Vista and Server 2008) cannot.