Project: Multi-File Price Report

A multi-file script is the next step after writing everything in one file. Real PHP projects split data, reusable functions, configuration, and entry-point scripts so each file has a clear job.

This mini-project builds a small price report from three files:

• data.php returns product data.
• functions.php contains reusable helpers.
• report.php loads the other files and prints the report.

The aim is not to create a framework. The aim is to practise file boundaries, require_once, returned arrays, small functions, and terminal output in a structure a junior developer would recognise in real work.

Give each file one responsibility

Start with the data file. A data file can return an array directly.

<?php

declare(strict_types=1);

return [
    ['name' => 'Notebook', 'price' => 499],
    ['name' => 'Pen', 'price' => 129],
    ['name' => 'Desk pad', 'price' => 1299],
];

This file does not print anything. It only returns data to the script that requires it.

Put helper functions in their own file

Formatting and calculation rules belong in a helper file when they are reused or when the entry script is becoming cluttered.

<?php

declare(strict_types=1);

function formatMoney(int $pennies): string
{
    return 'GBP ' . number_format($pennies / 100, 2);
}

function reportLine(array $product): string
{
    return $product['name'] . ': ' . formatMoney($product['price']);
}

These functions still use plain arrays because this track has not moved into object-oriented design yet. The important point is that the names describe the rules clearly.

Load files from the entry script

The entry script is the file you run from the terminal. It loads the functions, loads the data, and coordinates the output.

<?php

declare(strict_types=1);

require_once __DIR__ . '/functions.php';

$products = require __DIR__ . '/data.php';

foreach ($products as $product) {
    echo reportLine($product) . PHP_EOL;
}

// Prints:
// Notebook: GBP 4.99
// Pen: GBP 1.29
// Desk pad: GBP 12.99

Use __DIR__ so the include path is based on the file location, not the directory the command was run from. That prevents confusing path bugs.

Use require_once for functions

Function files should normally be loaded with require_once. If the same function file is required twice with require, PHP tries to declare the functions twice and the script fails.

<?php

declare(strict_types=1);

require_once __DIR__ . '/functions.php';
require_once __DIR__ . '/functions.php';

echo formatMoney(499);

// Prints:
// GBP 4.99

require_once makes repeated loading safe. It does not mean you should require files randomly; it means shared helper files are protected from accidental double loading.

Use require when you need returned data

A file required by PHP can return a value. This is common for simple configuration and small data files.

<?php

declare(strict_types=1);

$settings = require __DIR__ . '/settings.php';

echo $settings['currency'];

If settings.php returns ['currency' => 'GBP'], the required value is assigned to $settings.

Validate loaded data before reporting

Once data crosses a file boundary, check it before using it. A missing product name or negative price should not quietly appear in a report.

<?php

declare(strict_types=1);

function validateProduct(array $product): void
{
    if (trim($product['name'] ?? '') === '') {
        throw new InvalidArgumentException('Product name is required.');
    }

    if (($product['price'] ?? -1) < 0) {
        throw new InvalidArgumentException('Product price cannot be negative.');
    }
}

validateProduct(['name' => 'Notebook', 'price' => 499]);

echo 'Product accepted';

// Prints:
// Product accepted

This keeps the report honest. Bad data is caught near the place where the report is built.

Keep the entry script readable

A useful entry script is short and direct. It should show the flow without hiding important work.

<?php

declare(strict_types=1);

require_once __DIR__ . '/functions.php';

$products = require __DIR__ . '/data.php';
$total = 0;

foreach ($products as $product) {
    validateProduct($product);
    $total += $product['price'];

    echo reportLine($product) . PHP_EOL;
}

echo 'Total: ' . formatMoney($total) . PHP_EOL;

The entry script now reads like a small checklist: load helpers, load data, validate, print lines, print total.

What a good solution includes

A good multi-file script should use __DIR__ for reliable paths, require_once for helper files, require for returned data, small named helper functions, and validation before output.

Before moving on, make sure you can run the project with php report.php and explain which file owns the data, which file owns the helper rules, and which file coordinates the report.