Tambahkan script ini sebelum </body> di landing.php:

<script>
function trackClick(landing_id, button_type, url) {
    var xhr = new XMLHttpRequest();
    xhr.open('POST', '/track.php', true);
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xhr.send('landing_id=' + landing_id + '&button_type=' + button_type);
    setTimeout(function() {
        window.location.href = url;
    }, 300);
}
</script>

Dan ganti tombol-tombol di landing.php dari:
<a href="<?php echo $btn_login_url; ?>" class="btn-daftar" onclick="trackClick(...)">

Menjadi:
<a href="javascript:void(0)" onclick="trackClick(<?php echo $landing_id; ?>, 'login', '<?php echo $btn_login_url; ?>')" class="btn-daftar">
    <span><?php echo $btn_login_text; ?></span>
</a>
<a href="javascript:void(0)" onclick="trackClick(<?php echo $landing_id; ?>, 'daftar', '<?php echo $btn_daftar_url; ?>')" class="btn-daftar">
    <span><?php echo $btn_daftar_text; ?></span>
</a>
<a href="javascript:void(0)" onclick="trackClick(<?php echo $landing_id; ?>, 'livechat', '<?php echo $btn_livechat_url; ?>')" class="btn-daftar">
    <span><?php echo $btn_livechat_text; ?></span>
</a>