[man]strtotime/man will take the format you specified and return a UNIX timestamp suitable for [man]date/man which will output the date in any format you like.
If you know you will always get dd/mm/yyyy and just want to convert that to yyyy-mm-dd then you can just use [man]ereg_replace/man to tweak the string if you do something like
$in = '12/12/2002';
$out = ereg_replace('([0-9]+)/([0-9]+)/([0-9]+)', '\\3-\\2-\\1', $in);
I would definately recommend using strtotime() though since that will check that the date is not garbage like '93/16/2003'.
