echo '<td><form action="buy_item.php"><button>Buy</button></form></td>';

<?php
session_start();
$conn = mysqli_connect("localhost", "localhost", "localhost", "localhost");
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT price,username,amount FROM items,users WHERE username = '" .
$_SESSION['username'] . "' ";

$result = $conn->query($sql);
if ($result->num_rows > 0) {
while($row = $result->fetch_assoc()) {
   if   $row["amount"]  >= $row["price"]  {
      $newAmount =  $row["amount"]  - $row["price"] ;
        header("location:/quickshops/seller/tutoadd.php");

        }
        else
            {
                                echo "Records added successfully.";
      }
     }