Find Primes in Range

<!--Write a PHP script PrimesInRange.php that receives two numbers – start and end – from an input field and displays all numbers in that range as a comma-separated list. Prime numbers should be bolded. Styling the page is optional.-->

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8"/>
    <title>The Primes In Range</title>
    <style type="text/css">
        p {
            font-size: 18px;
            color: #555;
        }

        strong {
            font-weight: 800;
            color: #000;
        }
    </style>
</head>
<body>
<form method="post">
    <p>
        <label for="number-min">Starting Index:</label>
        <input type="number" name="start" id="number-min" min="1" required/>
        <label for="number-max">End:</label>
        <input type="number" name="end" id="number-max" min="2" required/>
        <input type="submit" value="Submit"/>
    </p>
</form>
<p>
    <?php
    if (!array_key_exists('start', $_POST) || !array_key_exists('end', $_POST) ||
        (int)$_POST['start'] >= (int)$_POST['end']
    ) {
        die('The Input Data is OUT of Range or Form is Empty!!!');
    }

    $start = (int)$_POST['start'];
    $end = (int)$_POST['end'];
    for ($i = $start; $i <= $end; $i++) {
        $isPrime = true;
        $maxDivider = (int)sqrt($i);
        for ($j = 2; $j <= $maxDivider; $j++) {
            if (($i % $j) === 0) {
                $isPrime = false;
                break;
            }
        }

        if ($isPrime) {
            echo "<strong>$i</strong>";
        } else {
            echo $i;
        }

        if ($i < $end) {
            echo ', ';
        }
    }
    ?>
</p>
</body>
</html>