<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "database_ps";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $conn->prepare("SELECT (COALESCE(a.price, 0) - COALESCE(f.price, 0)) AS total_price FROM tblcustomer c LEFT JOIN (SELECT CustomerID, SUM(Price) as price FROM tblappointment GROUP BY CustomerID ) a ON a.CustomerID = c.CustomerID LEFT JOIN (SELECT CustomerID, SUM(Price) as price FROM tblfinances GROUP BY CustomerID ) f ON f.CustomerID = c.CustomerID where c.CustomerID=3");
    $stmt->execute();

    $result = $stmt->setFetchMode(PDO::FETCH_ASSOC);
}
catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
}
echo $result;

$conn = null;
?>

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "database_ps";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $conn->prepare("SELECT (COALESCE(a.price, 0) - COALESCE(f.price, 0)) AS total_price FROM tblcustomer c LEFT JOIN (SELECT CustomerID, SUM(Price) as price FROM tblappointment GROUP BY CustomerID ) a ON a.CustomerID = c.CustomerID LEFT JOIN (SELECT CustomerID, SUM(Price) as price FROM tblfinances GROUP BY CustomerID ) f ON f.CustomerID = c.CustomerID where c.CustomerID=3");
    $stmt->execute();

    $result = $stmt->fetch(PDO::FETCH_ASSOC);
}
catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
}
echo $fetch["total_price"];

$conn = null;
?>