Ref T9846. See PHI48. For replacing text in the form "ABC" with "ABBC", the trimmer had a bug.
Added failing tests, fixed 'em.
That build failure might be "legitimate-ish" -- maybe a version difference.
PHP 7.0.0 - If string is equal to start characters long, an empty string will be returned. Prior to this version, FALSE was returned in this case.