get data depend on particular id using php and mysql - php

Question

i just create the simple cart app.in this i add products to the cart,from that i want to buy the one particular product.So i want to get and display product based on product id. In this, i had receiving the error.Can anyone help to solve this problem?
Inserting product to database:
<?php
Include('connect.php');
$productId=$_POST['productId'];
$productName=$_POST['productName'];
$productPrice=$_POST['productPrice'];
$quantity=$_POST['quantity'];
$sql=mysql_query("INSERT INTO products (productId,productName,productPrice,quantity)
VALUES ('$productId','$productName','$productPrice','$quantity')") or die(mysql_error());
if($sql){
header('Location:buy.php');
}
else
{
echo"Failed to add cart !";
}
?>
Adding products to cart:
<?php
include('connect.php');
$sql="SELECT * FROM buy";
$result = mysql_query($sql);
$myArray = array();
echo "<table style='width:50%;'>
<tr>
<th>Product Name</th>
<th>Product Price</th>
<th>Quantity</th>
</tr>";
$index = 0;
while($row = mysql_fetch_assoc($result))
{
$myArray[$index] = $row;
$index++;
echo "<tr>";
echo "<td>" . $row['productName'] . "</td>";
echo "<td>" . $row['productPrice'] . "</td>";
echo "<td>" . $row['quantity'] . "</td>";
echo "<td><a href='quantity.php'><button class='btn btn-default col-md- offset-2' value='submit'>Buy Now</button></a></td>" ;
echo "<td><button class='btn btn-danger' value='submit'>Buy Later</button> </td>" ;
echo "</tr>";
}
?>
choose product and display product based on id:
<?php
include('connect.php');
$productId=$_GET['productId'];
$sql="SELECT productName, productPrice, quantity FROM products WHERE productId=" . $productId . "";
$result = mysql_query($sql);
$myArray = array();
echo "<table style='width:50%;'>
<tr>
<th>Product Name</th>
<th>Product Price</th>
<th>Quantity</th>
</tr>";
$index = 0;
while($row = mysql_fetch_assoc($result))
{
$myArray[$index] = $row;
$index++;
echo "<tr>";
echo "<td>" . $row['productName'] . "</td>";
echo "<td>" . $row['productPrice'] . "</td>";
echo "<td>" . $row['quantity'] . "</td>";
echo "</tr>";
}
?>