Check that recurrent event, that happens same time (but can be repeated in different days) cross itself

Question

I have an event, that has start date, "2013-12-06 21:00:00", duration and repeat days bitmask. Problem is that user can specify duration > 24 hours and select repeat days that event will cross itself.

For example

1. Event duration is 25 hours, user selects to repeat it every day, so it will cross itself.
2. Event duration is 49 hours, user selects monday and wednesday, so event will cross itself again.

It is necessary to consider that validation should inclide event cross check in monday after sunday.

Could you please review my solution? perhaps it could be optimized somehow?

// Input params
$start = '2013-12-06 12:42:00';
$duration = 60 * 60 * 24 + 1; // specify duration
$days = '1000001'; // monday is at 0 position, sunday is at 6

// Loop internal vars
$dayLength = 60 * 60 * 24;
$loopStart = strtotime($start);
// had to take 8 days to check all week
$loopEnd   = $loopStart + ($dayLength * 8); 

$previousDateEnd = null;
for ($loop = $loopStart; $loop < $loopEnd; $loop += $dayLength) {

    $weekday = date('N', $loop) - 1;

    if ($days{$weekday}) {
        if (!is_null($previousDateEnd) && $loop < $previousDateEnd) {
            die('Event cross itself!');
        }
        $previousDateEnd = $loop + $duration;
    }
}

UPD: my code contains a bug, if i will try input params like that, it will not find event cross:

$start = '2013-12-06 12:42:00';
$duration = 60 * 60 * 24 * 2 + 1;
$days = '0001010'; // if one day is not just after another - it does not detect cross!

Answer 1

Optimized version, with fixed bug, described in question.

// Input params
$start = '2013-12-06 12:42:00';
$duration = 60 * 60 * 24 * 3;
$repeatDays = '0001001'; // monday is at 0 position, sunday is at 6

$requiredDaysOffset = ceil($duration / (60 * 60 * 24)) - 1;
for($i = 0; $i < 7; $i++) {
    if ($repeatDays[$i]) {
        echo 'index: ' . $i . ' ';

        // Check that next $requiredDaysOffset days are disabled
        $pos = $i;
        echo 'check next ' . $requiredDaysOffset . ' days: ';
        for($j = 0; $j < $requiredDaysOffset; $j++) {
            $pos = $pos < 6 ? ($pos + 1) : 0;
            echo "\npos: " . $pos . " ? " . $repeatDays{$pos};
            if ($repeatDays{$pos}) {
                echo " cross!";
            }

        }

        echo "\n";
    }
}