PHP timestamps and MySQL datetimes are not the same thing... it all depends on your column types.
There is a MySQL function called something like UNIXTIMESTAMP that will return a MySQL date as a timestamp and I know there's one for converting from UNIX to MySQL, but I don't know it off the top of my head. However, do some digging for UNIXTIMESTAMP and you're bound to find the 'reverse' function name.