that looks suspiciously like a mySQL timestamp. if it is you should use mySQL's DATE_FORMAT() function. however, if you must do this is PHP then...
$timestamp = '20041217192047';
$year = substr($timestamp, 0, 4);
$month = substr($timestamp, 4, 2);
$day = substr($timestamp, 6, 2);
$hour = substr($timestamp, 8, 2);
$minute = substr($timestamp, 10, 2);
$second = substr($timestamp, 12, 2);
$unix_timestamp = mktime($hour, $minute, $second, $month, $day, $year);
echo date('m/d/Y h:i:s a', $unix_timestamp);