visa 90 days in 180 days calculator

visa_free_days_left.php

<?php

$visitDates = [
    ['2020-06-16', '2020-06-30'],    
    ['2020-07-23', '2020-07-28'],    
    ['2020-10-09', '2020-11-04'],    
    ['2020-12-15', '2021-01-16'],    
    ['2021-01-30', '2021-02-26'],    
];

$visaDays = 90;
$visaWindowDays = 180;
$controlDate = (new DateTime('2021-04-06'))->modify(sprintf('-%d day', $visaWindowDays));
// var_dump($controlDate);

foreach($visitDates as $visitDate) {
    $leaveDate = new DateTime($visitDate[1]);
    
    if($leaveDate < $controlDate) {
        continue;
    }
    
    $enterDate = new DateTime($visitDate[0]);
    if($enterDate < $controlDate) {
        $enterDate = $controlDate;
    }
    
    $durationDays = $leaveDate->diff($enterDate)->days + 1;
    // var_dump($durationDays);
    $visaDays -= $durationDays;
}

var_dump($visaDays);