See: http://php.net/manual/en/migration71.other-changes.php

We are currently generating these errors in PHP 7.1. For example:

PHP Warning: in file /includes/functions_content.php on line 779: A non-numeric value encountered
PHP Warning: in file /includes/functions_content.php on line 780: A non-numeric value encountered
PHP Warning: in file /includes/functions_content.php on line 781: A non-numeric value encountered

The error above is because we are using a string, not a number to do bitwise calculations for, in this case the BBCODE bitfield value. This value should always be an int, but it is almost always being treated as a string, throughout the core.