Change date from input type date

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PHP

So I have this form with a input type date. users can choose a check in date and a check out date. this works but now I want to tinker into the input type date and make the dates that are available to book green and the dates that aren't have to be red. my question is: how do I do this?

this is an example of what I currently have

<pre>            <div class="information mb-3">
                    <div class="check in mb-3">
                        <label for="checkIn" class="form-label">Check in </label> 
                        <?php
                        $sql = "SELECT * FROM reserveringen";
                        $stmt = $conn->prepare($sql); 
                        $stmt->execute();
                        $result = $stmt->get_result();
                        while ($row = mysqli_fetch_assoc($result)) {
                            echo $row['start_datum']; //start date
                            echo $row['eind_datum']; //end date
                        }
                        ?> 
                        <input type="date" class="form-control"  id="checkInDate"/> 
                    </div>

What I have tried:

 I have been looking on the internet but I must not be able to explain myself the right way because it only tells me 
<pre><input type="date" class="form-control" id="checkOutDate"/> 

but I already have that.
I know I need a db where I have all the reserved dates and I have that already. I also already have a db connection I just don't understand how I should mix the two