You should read up on the DateTime classes, specifically DatePeriod and DateInterval:
$start = new DateTime('2014-01-03');
$interval = DateInterval::createFromDateString('4 weeks');
$end = new DateTime('2015-12-31');
$occurrences = new DatePeriod($start, $interval, $end);
foreach ($occurrences as $occurrence) {
echo $occurrence->format('Y-m-d') . PHP_EOL;
}
DatePeriod
takes a start date and a DateInterval
and allows you traverse over the object to get all dates within the boundaries using the given interval. The cut off can be either a set number of cycles (so the next 10 dates) or an end date (like above), even if the end date is not one of the dates the interval falls on (it will stop below it). Or you can use an 8601 interval notation string (which sounds so much fun, huh?), but I'm pretty shaky on that.