unable to display database content into specified field - php

Question

i am trying to create e-commerce website. neither it is showing error not providing coding result. trying to develop popup modal it is not showing results, modal popup is appearing but the echo codes which were put to retrieve database table value, for example this command is not working. how to find mistake?
<!-- The Modal -->
<?php
require_once '../core/init.php';
<!--this code is to retreive data from database thelewala and table products-->
$id = $_post['id'];
$id = (int)$id;
$sql = "SELECT * FROM products WHERE id = '$id'";
$result = $db->query($sql);
$product = mysqli_fetch_assoc($result);
<!-- this code is to retrieve data from database thelewala and table brand-->
$brand_id = $product['brand'];
$sql = "SELECT brand FROM brand WHERE id = '$brand_id'";
$brand_query = $db->query($sql);
$brand = mysqli_fetch_assoc($brand_query);
?>
<?php ob_start(); ?>
<div class="modal fade" id="details-modal">
<div class="modal-dialog">
<div class="modal-content">
<!-- Modal Header -->
<div class="modal-header">
<h4 class="modal-title"><?php echo $product['title']; ?></h4>
<button type="button" class="close" data-dismiss="modal">×</button>
</div>
<!-- Modal body -->
<div class="modal-body">
<div class="container-fluid">
<div class="row">
<div class="col-sm-6">
<div>
<img src="<?php echo $product['image']; ?>" alt="<?php echo $product['title']; ?>'" class="details img.responsive" />
</div>
</div>
<div class="col-sm-6">
<h4>Details</h4>
<p><?php echo $product['discription']; ?></p>
<hr />
<p>Price : Rs. <?php echo $product['price']; ?></p>
<p>Brand : <?php echo $brand['brand']; ?></p>
<form action="add_card.php" method="post">
<div class="form-group">
<div class="col-xs-3">
<lable for="quantity">Quantity</lable>
<input type="text" class="form-control" id="quantity" name="quantity" />
</div>
</div>
<div class="form-group">
<lable for="size">Size</lable>
<select name="size" id="size" class="form-control">
<option value=""></option>
<option value="32">32</option>
<option value="36">36</option>
<option value="XL">XL</option>
</select>
</div>
</form>
</div>
</div>
</div>
</div>
<!-- Modal footer -->
<div class="modal-footer">
<button type="button" class="btn btn-danger" data-dismiss="modal">Close</button>
<button type="submit" class="btn btn-warning" <span class="glyphicon glyphicon-shopping-cart"></spa>Add to Cart</button>
</div>
</div>
</div>
</div>
<?php echo ob_get_clean(); ?>