CVE-2026-24895: FrankenPHP's unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAME/PATH_INFO confusion) in FrankenPHP

FrankenPHP’s CGI path splitting logic improperly handles Unicode characters during case conversion. The logic computes the split index (for finding .php) on a lowercased copy of the request path but applies that byte index to the original path.

Because strings.ToLower() in Go can increase the byte length of certain UTF-8 characters (e.g., Ⱥ expands when lowercased), the computed index may not align with the correct position in the original string. This results in an incorrect SCRIPT_NAME and SCRIPT_FILENAME, potentially causing FrankenPHP to execute a file other than the one intended by the URI.