PHP calendar making booked days red

[220061]

hello! i'm making a calendar that shows which days have been booked and which are available.
In my db I have two important cols for this called: start_date and end_date (translation is start_datum and eind_datum)
Now I'm trying to make the days that are booked have the red booked label. the problem starts when I do this:
(written in d-m-y)
reservation is from 14-06-2022 till 18-06-2022
14-06-2022 = start_datum
18-06-2022 = eind_datum
those two dates turn red like they should be but all the dates in between don't change at all. I have tried to use a for loop to get this fixed but it's showing no difference. I'm out of idea's and was wondering if someone else knew what to do

        if(in_array($date, $bookings)){

            // misschien lukt het nog met de tutorials erbij sze hebben er namelijk nog mee rgemaakt
           print_r($bookings);
        //     echo "<br>";
        //     echo $date;
        //     echo "<br>";
        $timestampStart_datum = strtotime($start_datum);
        //schets voor mktime()
        //mktime(Hour, minute , second, month, day, year);
        $testing = mktime(0, 0, 0, $timestampStart_datum);
        echo $testing, "<---- dit is de variable testing";
        echo "<br>";
        $timestampEind_datum = strtotime($eind_datum);
        $testingEind = mktime(0, 0, 0, $timestampStart_datum);
        echo $testingEind, "<---- dit is de variable testing eind";
        echo "<br>";
        $seconds = 24 * 60 * 60;
        echo "<br>";
        // echo $start_datum;
        // echo "<br>";
        // echo $eind_datum;
        // echo "<br>";
        if($start_datum < $date){
            echo "testing";
        }else{
            echo "testing gone wrong????";
        }
        echo "<br>";
        echo $currentDay . "<--- dit is de current day";
        echo "<br>";
        echo $testing . "<--- dit is de testing day";
        echo "<br>";
        echo $testingEind . "<--- dit is de testingEind day";
        echo "<br>";
        echo $today . "<--- dit is de today day";
        echo "<br>";
        echo $date . "<--- dit is de date day";
        echo "<br>";
            for ($date = $testing; $date <= $testingEind; $date++) {
                //$next = date("F j, Y", $date);
             $calendar.="<td class='$today'><h4>$currentDay</h4><a class='btn btn-danger btn-xs'>booked</a></td>";
            }
           // $calendar.="<td class='$today'><h4>$currentDay</h4><a class='btn btn-danger btn-xs'>booked</a></td>";
        }else{
            $calendar.="<td class='$today'><h4>$currentDay</h4><a class='btn btn-success btn-xs'>Beschrikbaar</a></td>";
        }
        $currentDay++;
        $dayOfWeek++;

    }