Fix convertPhpToIsoFormat

Fixes bug [ZF-12141](http://framework.zend.com/issues/browse/ZF-12141)

Thiis new code has been used in production on many websites running concrete5 without any problems for two years (see the [concrete5 patched version](https://github.com/concrete5/concrete5/blob/master/web/concrete/libraries/3rdparty/Zend/Locale/Format.php#L736))