<?php
$servername = "localhost";
$username = "aaa";
$password = "aaaa";
$dbname = "mydb";


$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "SELECT sum(base_grand_total) as total FROM `sales_order` WHERE status='processing_closed'";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo  $row["total"];
    }
} else {
    echo "0 results";
}
$conn->close();
?>