updating a database more then once php

I have a total amount. when i sell am item i want to add that amount to my total. e.g i have 100.00, i sell a snake for 100.00 so my total should be 200.00. I have it working so it adds the 100.00 but if i want to sell two snakes it will stay at 200.00 it wont add any amount from the second snake. using this to update my database.

if(isset($_POST['go'])
              { 
 $query = "UPDATE  users  SET amount=$current WHERE email = '".$_SESSION['email']."'";

}

           if(isset($_POST['go'])      //edit 17.06.2022
           {  
                    $current = $amount+100;


           }

1. You need to do the calculation before you use the calculated amount, which is $current
2. You need to execute the sql statement

if(isset($_POST['go'])
{
 $current = $amount+100;
 $query = "UPDATE  users  SET amount=$current WHERE email ='".$_SESSION['email']."'";
$result=$db->exec($query);
}  
      

$db is from the connection file below

<?php

  $db=new PDO('mysql:host=localhost;dbname=testdb;charset=utf8','root','');
    $db->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
?>

Add the below line before interacting with the DB.

require('connection.php');

When you get it to run, I suggest looking at prepared statements to prevent SQL injection vulnerabilities on your app