Summary: Fixes T5545. We assume `strlen()` returns the number of bytes in a string, which is the normal behavior (and the documented behavior). There's a config option, `mbstring.func_overload`, which silently calls mb_strlen() instead. This may return some other result, might fail, etc., and there's no way to get the byte length of a string if this option is set. If this option is set, fatal immediately. Nothing good can ever come of it. Test Plan: {F173990} Reviewers: btrahan, chad Reviewed By: chad Subscribers: epriestley Maniphest Tasks: T5545 Differential Revision: https://secure.phabricator.com/D9811