Undefined variable in PHP and MySQL

There are a couple of issues in your code, firstly you appear to be binding (bind_param) a variable ($param_id) which isn’t yet defined which should error, but ultimately will return zero rows from your SQL query.

Secondly, where it is in the provided code, the $id_customer variable will only be defined in certain circumstances. There are 4 if statements which need to be true before $id_customer is declared. Should any of those 4 statements be false, it won’t be assigned a value. So when you try to echo it at the end of your script, it won’t be defined. Hence the error message.

It’s difficult to provide you with an answer without understanding exactly what your script wants to achieve. But in short, $id_customer will only have a value inside the if statement if ($result->num_rows == 1) { /** ... **/ }.

The following will provide you with the desired output:

<?php
if (isset($_GET["id_customer"]) && !empty(trim($_GET["id_customer"]))) {
    require_once "../connectDB.php";

    $sql = "SELECT * FROM Customers 
            INNER JOIN Orders
            ON Customers.id_customer = Orders.id_customer
            WHERE Customers.id_customer = ?";

    if ($stmt = $mysqli->prepare($sql)) {

        // Set parameters
        // Define $param_id before use, otherwise you'll be binding an undefined or null variable = no results + error
        $param_id = trim($_GET["id_customer"]);

        // Bind variables to the prepared statement as parameters
        $stmt->bind_param("i", $param_id);

        // Attempt to execute the prepared statement
        if ($stmt->execute()) {
            $result = $stmt->get_result();

            if ($result->num_rows == 1) {
                /* Fetch result row as an associative array. Since the result set
                contains only one row, we don't need to use while loop */
                $row = $result->fetch_array(MYSQLI_ASSOC);

                // Retrieve individual field value
                $id_customer = $row["id_customer"];
                $fn_customer = $row["fn_customer"];
                $ln_customer = $row["ln_customer"];
                $email_customer = $row["email_customer"];
                $id_order = $row["id_order"];
                $order_dt = $row["order_dt"];
                $order_total = $row["order_total"];
                // $id_customer will be defined here
                ?>
                   <p><?php echo $id_customer; ?></p>
                <?php
            }
            // $id_customer MAY NOT be defined here
        }
        // $id_customer MAY NOT be defined here
    }
    // Close statement
    $stmt->close();
        
    // Close connection
    $mysqli->close();
    
    // $id_customer MAY NOT be defined here
}

// $id_customer MAY NOT be defined here

?>