how to update quantity to calculate total price from price in php

<?php
	$pro_id = $_COOKIE['product_id'];   
	$pro_id = explode(',', $pro_id);
	$qua = 1;
	foreach ($pro_id as $val) {
		$ans = "SELECT * FROM wm_products WHERE pro_id='$val'";
		$result = $conn->query($ans);
		while($row = $result->fetch_assoc()){
		?>
			<tr class="cart_item">
				<td>
					<a href="single-product.html"><img src="products_images/<?php echo $row['pro_img']; ?>" alt="" width="180" height="180"></a>
				</td>
				<td>
					<a href="single-product.html"><?php echo $row['pro_name']; ?></a>
				</td>
				<td>
					<span class="amount">Rs. <?php echo $row['pro_price']; ?></span>
				</td>
				<td>
					<div class="quantity buttons_added">
					<form action="" method="post">
						<input type="submit" class="minus" name="minus" value="-">				
						<input type="text" size="4" class="input-text qty text" title="Qty" value="<?php echo $qua;?>" name="" max="29" min="0" step="1" >
						<input type="submit" class="plus" name="plus" value="+">
					</form>					
					</div>
				</td>
				<td>
					<span class="amount">Rs. <?php echo number_format($row['pro_price'] * $qua); ?></span>
				</td>
				<td>
					<a class="remove remove_cart" href="#" dataid="<?php echo $row['pro_id']; ?>">×</a>
				</td>
			</tr>
		<?php 
		}                                   
	}
?>