PHP - Reducing an independent record stock value by 1 - php

Question

I'm currently doing a school project and I'm using dreamweaver along with a backend database using phpMyAdmin.
Now, what i need to do is, when I click the button, it will reduce the stock column value in the "products" table by 1.
However there are different products in the table. Shown below:
http://i.stack.imgur.com/vLZXQ.png
So lets say, A user is on the game page for "Destiny" and clicks on the Buy now button, how can i make it reduce the stock level by one, but only for the Destiny record and not for the Fifa 15 column. So Destiny stock becomes 49, but Fifa stays 50. Will i just need to make each button have a different script or?
Currently, I made a button in the page, which links to an action script, but im not sure what sort of code i will be using.
Thank you

Answer 1

xNeyte is giving you some good advice, but it comes across to me that you - Xrin - are completely new to programming database contents with PHP or similar?
So some step by steps:
MYSQL databases should be connected with one of two types of connection - PDO and MySQLi_ . MySQL databases will also always work using the native MySQL but as xNeyte already mentioned - this is deprecated and highly discouraged .
So what you have is you pass your information to the PHP page, so your list of games is on index.php and your working page that will update the number of games ordered would be update.php, in this example.
The Index.php file passes via anchor link and $_GET values (although I highly recommend using a php FORM and $_POST as a better alternative), to the update.php page, which needs to do the following things (in roughly this order) to work:
Update.php
Load a valid database login connection so that the page can communicate with the database
Take the values passed from the original page and check that they are valid.
establish a connection with the database and adjust the values as required.
establish the update above worked and then give the user some feedback
So, step by step we'll go through these parts:
I am going to be a pain and use MySQLi rather than PDO - xNeyte used PDO syntax in his answer to you and that is fully correct and various better than MySQLi, for the sake of clarity and your knowledge of MySQL native, it may be easier to see/understand what's going on with MySQLi.
Part 1:
Connection to the database.
This should be done with Object Orientated - Classes,
class database {
private $dbUser = "";
private $dbPass = ""; //populate these with your values
private $dbName = "";
public $dbLink;
public function __construct() {
$this->dbLink = new mysqli("localhost", $this->dbUser, $this->dbPass, $this->dbName);
}
if (mysqli_connect_errno()) {
exit('Connect failed: '. mysqli_connect_error());
}
if ( ! $this->dbLink )
{
die("Connection Error (" . mysqli_connect_errno() . ") "
. mysqli_connect_error());
mysqli_close($this->dbLink);
}
else
{
$this->dbLink->set_charset("UTF-8");
}
return true;
} //end __construct
} //end class
The whole of the above code block should be in the database.php referenced by xNeyte - this is the class that you call to interact with the database.
So using the above code in the database.php object, you need to call the database object at the top of your code, and then you need to generate an instance of your class:
include "database.php"; ////include file
$dataBase = new database(); ///create new instance of class.
Now When you write $dataBase->dbLink this is a connection to the database. If you do not know your database connection use the details PHPMyAdmin uses, it carries out its tasks in exactly the same way.
Sooo
Part 2:
That is that your database connection is established - now you need to run the update: First off you need to check that the value given is valid:
if (is_numeric($_GET['id']) && $_GET['id'] >0 ){
$id = (int)$_GET['id'];
}
This is simple code to check the value passed from the link is a integer number. Never trust user input.
It is also a good idea never to directly plug in GET and POST values into your SQL statements. Hence I've copied the value across to $id
Part 3:
$sql = "UPDATE <TABLE> SET STOCK = STOCK-1 WHERE Product_ID = ? LIMIT 1";
The table name is your table name, the LIMIT 1 simply ensures this only works on one row, so it will not effect too many stocked games.
That above is the SQL but how to make that work in PHP:
first off, the statement needs to be prepared, then once prepared, the value(s) are plugged into the ? parts (this is MySQLi syntax, PDO has the more useful :name syntax).
So:
include "database.php"; ////include file
$dataBase = new database(); ///create new instance of class.
if (is_numeric($_GET['id']) && $_GET['id'] >0 ){
$id = (int)$_GET['id'];
$sql = "UPDATE <TABLE> SET STOCK = STOCK-1 WHERE Product_id = ? LIMIT 1";
$update = $dataBase->dbLink->prepare($sql);
$update->bind_param("i",$id);
$update->execute();
$counter = $update->affected_rows;
$update->close();
//////gap for later work, see below:
}
else
{
print "Sorry nothing to update";
}
There's probably quite a lot going on here, first off the bind_param method sets the values to plug into the SQL query, replacing the ? with the value of $id. The i indicates it is meant to be an Integer value. Please see http://php.net/manual/en/mysqli-stmt.bind-param.php
The $counter value simply gets a return of the number of affected rows and then something like this can be inserted:
if ($counter > 0 ){
print "Thank you for your order. Stock has been reduced accordingly.";
}
else {
print "Sorry we could not stock your order.";
}
Part 4
And finally if you wish you can then just output the print messages or I tend to put the messages into a SESSION, and then redirect the PHP page back.
I hope this has helped a bit. I would highly recommend if you're not used to the database interactions in this way then either use PDO or MySQLi but do not combine the two, that will cause all sorts of syntax faults. Using MySQLi means that everything you know MySQL can do, is done better with the addition of the letter "i" in the function call. It is also very good for referencing the PHP.net Manual which has an excellent clear detailed examples of how to use each PHP function.

Answer 2

The best is to set a link on each button with the ID of your game (1 for destiny, 2 for Fifa15).
Then your script which the user will launch by clicking will be :
<?php
include('database.php'); // your database connection
if($_GET['id']) {
$id=$_GET['id'];
} else throw new Exception('Invalid parameter');
$statement = myPDO::getInstance->prepare(<<<SQL
UPDATE TABLE
SET STOCK = STOCK-1
WHERE Product_id = :id
SQL
);
$statement->execute(array(":id" => $id));
This script will do the job