I suspect jpmoriarty is right: 12/04/2003 is ambiguous because Americans would read it as 4th December, while everyone else would read it as 12th April. So, long-term answer: don't use ambiguous date formats. ISO recommends yyyy-mm-dd.
Short term, use explode() on the '/' character, then mktime to put the bits together into a timestamp, and then use date() on the result.