With PHP subtract a quantity from an mysql table - php

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.
read value from url: http://mypage/getdata.php?value=caffe
read value from database table caffe
subtract -1 from the table
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