Reusing PHP Files

Imagine a site with ten pages. Each needs the same helper to escape output. If you copy the helper into every page, fixing a bug means editing ten files.

Put it in one file, and load that file everywhere.

Step 1: a shared functions file

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<?php
declare(strict_types=1);

function h(string $text): string
{
    return htmlspecialchars($text, ENT_QUOTES, "UTF-8");
}

function money(float $amount): string
{
    return "Rs. " . number_format($amount, 2);
}

Step 2: use it from any page

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<?php
require_once __DIR__ . "/functions.php";

echo h("<b>Tom & Jerry</b>") . "\n";
echo money(1499.5);

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<b>Tom & Jerry</b>
Rs. 1,499.50

Now a bug fix or a change happens in one place.

A simple project layout

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my-site/
├── public/            <- the only folder the web server exposes
│   ├── index.php
│   └── contact.php
├── includes/
│   ├── functions.php
│   └── config.php
└── templates/
    ├── header.php
    └── footer.php

Each public page starts with:

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<?php
require_once __DIR__ . "/../includes/functions.php";

Files outside public/ cannot be opened directly from a browser. That protects your configuration files and code.

What belongs in a shared file

• Helper functions (escaping, formatting, redirects).
• Configuration.
• Class definitions (until you use an autoloader).
• Page parts (header, footer, navigation).

What does not belong

• Code that runs a page-specific action. Keep that in the page itself.
• Secrets in a file inside the web folder.

Include order matters

A file must be loaded before you use what it defines. Load configuration first, then functions, then anything that depends on them.

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<?php
require_once __DIR__ . "/config.php";
require_once __DIR__ . "/functions.php";
require_once __DIR__ . "/database.php";   // uses config values