Can you help me writing a PHP program to track invoices?
Yes, I can help you write a PHP program to track invoices. Here's a basic outline of how you can go about writing this program:

    First, you'll need to set up a database to store the invoice information. You can use a relational database management system like MySQL, and create tables to store information such as the invoice number, date, customer name, items on the invoice, and the amount.

    Next, you'll need to create a user interface for the program. This can be done using HTML and CSS to create a form where you can enter the invoice information. You can use PHP to process the form data and store it in the database.

    You'll also need to write code to retrieve the invoice information from the database and display it to the user. This can be done using PHP and SQL queries to retrieve the data and display it in a table or list format.

    Finally, you'll want to add some functionality to update and delete invoices. You can add buttons to the user interface that allow you to update or delete invoices, and write PHP code to process these actions.

Here's a basic example of what the code might look like:

php
```
<?php
// Connect to the database
$db = mysqli_connect('localhost', 'username', 'password', 'database');

// Check if the form has been submitted
if (isset($_POST['submit'])) {
  // Retrieve the form data
  $invoice_number = $_POST['invoice_number'];
  $date = $_POST['date'];
  $customer_name = $_POST['customer_name'];
  $item = $_POST['item'];
  $amount = $_POST['amount'];

  // Insert the data into the database
  $query = INSERT INTO invoices (invoice_number, date, customer_name, item, amount)
            VALUES ('$invoice_number', '$date', '$customer_name', '$item', '$amount');
  mysqli_query($db, $query);
}

// Retrieve the invoices from the database
$query = SELECT * FROM invoices;
$result = mysqli_query($db, $query);

// Display the invoices
echo '<table>';
echo '<tr><th>Invoice Number</th><th>Date</th><th>Customer Name</th><th>Item</th><th>Amount</th></tr>';
while ($invoice = mysqli_fetch_array($result)) {
  echo '<tr>';
  echo '<td>' . $invoice['invoice_number'] . '</td>';
  echo '<td>' . $invoice['date'] . '</td>';
  echo '<td>' . $invoice['customer_name'] . '</td>';
  echo '<td>' . $invoice['item'] . '</td>';
  echo '<td>' . $invoice['amount'] . '</td>';
  echo '</tr>';
}
echo '</table>';

// Close the connection to the database
mysqli_close($db);
?>
```
This is just a basic example, and there is much more you can do to build out a more sophisticated
What would you recommend for the database schema?   I want to store customer information such as their name, address and contact details and track which invoices have been paid and which are overdue.
Here's a suggested database schema to store customer information and track invoices:

```sql
CREATE TABLE customers (
  customer_id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(255),
  address TEXT,
  phone VARCHAR(255),
  email VARCHAR(255)
);

CREATE TABLE invoices (
  invoice_id INT AUTO_INCREMENT PRIMARY KEY,
  customer_id INT,
  invoice_number VARCHAR(255),
  date DATE,
  item VARCHAR(255),
  amount DECIMAL(10, 2),
  paid BOOLEAN DEFAULT 0,
  FOREIGN KEY (customer_id) REFERENCES customers (customer_id)
);
```
In this schema, we have two tables: customers and invoices. The customers table stores information about each customer, including their name, address, phone number, and email address. The invoices table stores information about each invoice, including the customer it is associated with, the invoice number, date, item, and amount. Additionally, we track whether the invoice has been paid using a paid field, which is a boolean value (0 for not paid, 1 for paid).

The customer_id field in the invoices table is a foreign key that references the customer_id field in the customers table, indicating the relationship between the two tables. This way, you can retrieve all the invoices for a given customer by joining the two tables on the customer_id field.