<?php echo ($id == $kategori->id) ? "selected" : ""; ?>

<select class="form-control keskinKenar col-2" name="category_id[]">
<?php foreach($categories as $category): ?>
<option <?php echo ($_GET['id'] == $category->id) ? "selected" : ""; ?>><?=$category['category_name']?></option>
<?php endforeach; ?>
</select>