With PHP subtract a quantity from an mysql table

Question

I am trying to do a php code that subtract -1 of quantity stored in a mysql table named ''caffe''. The table start with 10 and if php is executed with value caffe a quantity in table will be updated. Will be good some lines of code so i can understand how it work.

1. read value from url: http://mypage/getdata.php?value=caffe
2. read value from database table caffe
3. subtract -1 from the table
4. update the value of mysql table caffe

---

<?
$servername = "localhost";
$username = "xxx";
$password = "";
$database = "my_ufficina";


//creating a new connection object using mysqli 
$conn = new mysqli($servername, $username, $password, $database);

//if there is some error connecting to the database
//with die we will stop the further execution by displaying a message causing the error 
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}

//if everything is fine

$txt = $_GET['value'];

if ($txt == caffe)
$count = 1;

echo $count;

Answer 1

I would try something like that :

if ($txt === 'caffe') {

    // get Caffe value :

    try {
        $currentCaffeValue = $conn->query("SELECT yourCaffeValue FROM yourTable");
    } catch (Exception $e) {
        echo 'db read failed : ', $e->getMessage();
    }

    // do your stuff then update Caffe value :

    try {
        $conn->query("UPDATE yourTable SET yourCaffeValue = yourCaffeValue - 1");
    } catch (Exception $e) {
        echo 'db update failed : ', $e->getMessage();
    }
}

And by the way, you should sanitize values from $_GET before using it : http://php.net/manual/en/function.filter-input.php