I have just put a pull request together to make some changes to base_convert, the changes are as follows:
1, Allow conversion of negative numbers (fixes https://bugs.php.net/bug.php?id=55393)
2, Raise a notice when passing in invalid chars as input, currently they are just silently ignored
Part of the negative number change causes a BC break due to needing to use a "zend_long" instead of a "zend_ulong" this breaks a number of tests, but allows negative numbers to be represented correctly.
The PR is here https://github.com/php/php-src/pull/3911/files
I look forward to hearing thoughts on this