$query3 = 'SELECT users.first_name,users.last_name,users.phone, retailer.date, SUM(retailer.basket_value), retailer.time,retailer.location,retailer.type_of_payment
FROM users ,retailer WHERE users.user_id="'.$id_user.'"
AND users.user_id=retailer.user_id GROUP BY users.user_id';

$result3 = mysql_query($query) or die(mysql_error());

// Print out result
while($row = mysql_fetch_array($result3)){
echo "Total ". $row['user_id']. " = $". $row['SUM(basket_value)'];
echo "<br />";
}

$query3 = 'SELECT users.user_id,users.first_name,users.last_name,
                  users.phone, retailer.date,
                  SUM(retailer.basket_value) as total_sum,
                  retailer.time,retailer.location,retailer.type_of_payment
           FROM users ,retailer WHERE users.user_id="'.$id_user.'"
           AND users.user_id=retailer.user_id
           GROUP BY users.user_id,users.first_name,users.last_name,
                    users.phone, retailer.date,retailer.time,retailer.location,
                    retailer.type_of_payment '

$result = $mysqli->query($query3);

while ($row = $result->fetch_row()) {
        echo "Player: ".$row['user_id']." total: ".$row['total_sum'];
    }

SELECT SUM(retailer.basket_value) as total_sum
FROM retailer

SELECT users.first_name,
    users.last_name,users.phone, retailer.date,

    SUM(retailer.basket_value) as total_sum , retailer.time,
retailer.location,retailer.type_of_payment

    FROM users ,retailer WHERE users.user_id="'.$id_user.'"
        AND users.user_id=retailer.user_id

group by users.first_name,
    users.last_name,users.phone, retailer.date, retailer.time,
retailer.location,retailer.type_of_payment

SELECT u.first_name, u.last_name, u.phone, SUM(r.basket_value) as total_sum
FROM users u
JOIN retailers r
  ON u.user_id = r.user_id
WHERE u.user_id="'.$id_user.'"
GROUP BY u.user_id

$query3 = 'SELECT SUM(retailer.basket_value) as total_sum
FROM users ,retailer WHERE users.user_id="'.$id_user.'"
AND users.user_id=retailer.user_id';