Iterating over elements and applying marker class to achieve nth-of-class selection

gistfile1.phtml

<?php
$elements = array(
    array(
        'id' => 1,
        'is_test' => false
    ),
    array(
        'id' => 2,
        'is_test' => true
    ),
    array(
        'id' => 3,
        'is_test' => false
    ),
    array(
        'id' => 4,
        'is_test' => true
    ),
    array(
        'id' => 5,
        'is_test' => false
    )
);

$test_count = 0;
?>

<?php foreach($elements as $element): ?>
<div class="<?php 
    if ($element['is_test']){
        $test_count += 1;
        echo 'test ';
        
        if ($test_count % 2 == 0) {
            echo 'marker ';
        }
    } 
    ?>">
    Test <?php echo $element['id']; ?>
</div>
<?php endforeach ?>