🏠 PHP Variables & Constants: Your Data Storage Boxes

Imagine you have a magic house with different types of boxes to store your toys and treasures. Some boxes you can open and change what’s inside. Others are locked forever with special treasures that never change. That’s exactly how PHP stores information!

📦 What is a Variable?

A variable is like a labeled box where you put things. You can:

• Put something in the box
• Look at what’s inside
• Replace it with something new

In PHP, every variable starts with a dollar sign $ — think of it as a little price tag on your box!

$myToy = "Teddy Bear";
$age = 8;
$hasCandy = true;

What’s happening?

• $myToy holds the word “Teddy Bear”
• $age holds the number 8
• $hasCandy holds true (yes!)

🏷️ Variable Naming Rules

Just like you can’t name your pet “123Cat!” (weird, right?), PHP has rules for naming boxes:

✅ The Rules

// ✅ GOOD names
$userName = "Alex";
$_secretCode = 1234;
$player1Score = 100;

// ❌ BAD names (will break!)
// $2fast = "error";
// $my-name = "nope";
// $hello world = "spaces!";

🎯 Pro Tip: Use Descriptive Names!

// Hard to understand
$x = 25;

// Easy to understand
$playerAge = 25;

🌍 Variable Scope: Where Can Your Box Be Seen?

Imagine your house has different rooms. Some toys stay in your bedroom only. Others are in the living room where everyone can see them.

🏠 Three Types of Scope

graph TD
    A[🌍 PHP Scope Types] --> B[🏠 Local<br/>Inside function only]
    A --> C[🌐 Global<br/>Outside functions]
    A --> D[⚡ Static<br/>Remembers value]

1️⃣ Local Scope — Your Bedroom

Variables inside a function stay inside that function.

function sayHello() {
    $greeting = "Hi!";  // Local
    echo $greeting;     // Works!
}
sayHello();
// echo $greeting; ❌ Error! Can't see it here

2️⃣ Global Scope — The Living Room

Variables outside functions need special permission to enter.

$familyName = "Smith";  // Global

function introduce() {
    global $familyName;  // Ask permission!
    echo "We are the " . $familyName;
}
introduce();  // "We are the Smith"

3️⃣ Static Scope — The Memory Box

Normally, local variables forget everything after the function ends. Static variables remember!

function countVisits() {
    static $count = 0;  // Remembers!
    $count++;
    echo "Visit #" . $count;
}
countVisits();  // Visit #1
countVisits();  // Visit #2
countVisits();  // Visit #3

🦸 Superglobals: The Superhero Boxes

These are special boxes that PHP creates for you. They work EVERYWHERE — no permission needed! They always start with $_.

graph LR
    A[🦸 Superglobals] --> B["$_GET<br/>URL data"]
    A --> C["$_POST<br/>Form data"]
    A --> D["$_SESSION<br/>User session"]
    A --> E["$_COOKIE<br/>Browser cookies"]
    A --> F["$_SERVER<br/>Server info"]
    A --> G["$_FILES<br/>Uploaded files"]
    A --> H["$_ENV<br/>Environment"]
    A --> I["$_REQUEST<br/>GET + POST"]
    A --> J["$GLOBALS<br/>All globals"]

📝 Quick Examples

// Getting data from a URL like:
// mysite.com?name=Alex
echo $_GET['name'];  // "Alex"

// Getting form data
echo $_POST['email'];

// Server information
echo $_SERVER['HTTP_HOST'];  // Your domain

// Session data (like a backpack)
$_SESSION['user'] = "Alex";

🎒 Think of It Like…

🔒 Constants: The Treasure Chest

Unlike variables, constants NEVER change. Once you set them, they’re locked forever — like carving your name in stone!

📌 Two Ways to Create Constants

// Method 1: define()
define("SITE_NAME", "My Awesome Site");
define("MAX_PLAYERS", 100);

// Method 2: const (inside classes too!)
const PI = 3.14159;
const GAME_VERSION = "2.0";

⚡ Key Differences from Variables

define("GRAVITY", 9.8);

function calculateFall() {
    // No 'global' needed!
    return GRAVITY * 2;  // Works directly!
}

✨ Magic Constants: The Shape-Shifters

These special constants change their value depending on where you use them! They have double underscores on both sides: __NAME__

graph LR
    A[✨ Magic Constants] --> B["__LINE__<br/>Current line number"]
    A --> C["__FILE__<br/>Full file path"]
    A --> D["__DIR__<br/>File directory"]
    A --> E["__FUNCTION__<br/>Function name"]
    A --> F["__CLASS__<br/>Class name"]
    A --> G["__METHOD__<br/>Class::method"]
    A --> H["__NAMESPACE__<br/>Namespace name"]
    A --> I["__TRAIT__<br/>Trait name"]

🔮 See Them in Action!

<?php
// In file: /var/www/app/game.php

echo __LINE__;     // 5 (this line number)
echo __FILE__;     // /var/www/app/game.php
echo __DIR__;      // /var/www/app

function battle() {
    echo __FUNCTION__;  // "battle"
}

class Hero {
    public function attack() {
        echo __CLASS__;   // "Hero"
        echo __METHOD__;  // "Hero::attack"
    }
}

🎯 Why Are They Useful?

Perfect for debugging (finding problems):

function logError($message) {
    echo "Error on line " . __LINE__;
    echo " in file " . __FILE__;
    echo ": " . $message;
}

🎮 Quick Summary: Your PHP Storage System

🚀 You Did It!

Now you understand how PHP stores data:

• Variables = Changeable labeled boxes
• Naming Rules = How to properly name your boxes
• Scope = Where your boxes can be seen
• Superglobals = Superhero boxes that work everywhere
• Constants = Locked treasure chests
• Magic Constants = Shape-shifting special values

You’re ready to store and manage data like a PHP pro! 🎉