PHP Variables

PHP Variables, Scope, & Naming Rules

Variables in PHP are containers for storing data values (strings, integers, arrays, objects). All PHP variables begin with a dollar sign $ followed by the variable name.

Variable Naming Rules & Memory Model

1. Variable names must start with a letter or an underscore _.
2. Variable names cannot start with a number.
3. Variable names can only contain alphanumeric characters and underscores (A-z, 0-9, and _).
4. Variable names are case-sensitive ($age and $AGE are two separate variables).

$validVar = "Allowed";
$_user_id = 101;
//$2ndRank = "Invalid!"; // Syntax Error!

Variable Scope Types

PHP has three distinct variable scopes: local, global, and static.

flowchart TD
    A["PHP Variable Scopes"] --> B["Local Scope"]
    A --> C["Global Scope"]
    A --> D["Static Scope"]
    B --> E["Declared inside function; inaccessible outside"]
    C --> F["Declared outside function; requires 'global' keyword or $GLOBALS inside"]
    D --> G["Declared inside function; retains state across calls"]

Practical Code Example

<?php
declare(strict_types=1);

// Global Scope Variable
$appName = "Kodersolution Engine";

function displayCounter(): void
{
    // Accessing Global Variable via $GLOBALS superglobal
    echo "App Name: " . $GLOBALS['appName'] . "
";

    // Static Variable: retains value across multiple function calls
    static $visitCount = 0;
    $visitCount++;

    // Local Scope Variable
    $localMessage = "Function Call #" . $visitCount;
    echo $localMessage . "
";
}

displayCounter(); // Output: Call #1
displayCounter(); // Output: Call #2
displayCounter(); // Output: Call #3

// Attempting to access $localMessage here causes Undefined Variable Error!

Best Practices

• Use camelCase or snake_case Consistently: Stick to standard camelCase ($userEmail) or snake_case ($user_email) throughout project repositories.
• Avoid Overusing Global Variables: Passing values via parameters or dependency injection is safer and cleaner than referencing global variables.
• Use static for Cached State within Functions: Ideal for memoization or counter logic that persists across function invocations within the request execution.

Self-Check Challenge

Create a function trackDownloads() containing a static $count = 0; variable that increments and prints the download count every time the function is called!