bootstrap.php has this code:

// If we are on PHP >= 6.0.0 we do not need some code

if (version_compare(PHP_VERSION, '6.0.0-dev', '>='))

We changed it to check against 5.4.0 in startup.php, same change should be made here I suppose.

Another occurrence is in phpBB/includes/request/type_cast_helper.php which is in develop only.