PHP Variables

Variable Basics

Variable Declaration

PHP variables are created the moment you assign a value to them. Unlike some languages, you don’t need to explicitly declare variables before use.

They start with the $ symbol followed by the variable name.

// Variables are created the moment you assign a value
$message = "Hello, World!";
$count = 42;
$price = 19.99;
$isActive = true;
$items = array("apple", "banana", "orange");

The dollar sign distinguishes variables from other programming elements like function names or keywords. Without it, PHP wouldn’t know whether you’re referring to stored data or trying to use a command.
The dollar sign serves as a signal flag that says “container ahead!”
It’s PHP’s way of immediately telling you “this is a variable” when you read the code.

Variable Naming Rules

Must start with $ followed by a letter or underscore
Can contain letters, numbers, and underscores
Case-sensitive ($name and $Name are different)
Cannot start with a number

// Valid variable names
$userName = "john";
$user_name = "john";
$_userName = "john";
$userName123 = "john";

// Invalid variable names
// $123user = "john";     // Cannot start with number
// $user-name = "john";   // Hyphens not allowed
// $user name = "john";   // Spaces not allowed

The Human-Readable Rules for Better Code

<?php
// Good variable names tell a story
$customer_email = "alice@email.com";    // Anyone reading this knows exactly what data this contains
$monthly_salary = 5000;                 // Clear time period and purpose
$is_logged_in = true;                   // Boolean variables often start with "is_" to show true/false nature

// Poor variable names create confusion
$x = "alice@email.com";                 // What does 'x' mean? You'll forget in a week
$data = 5000;                           // What kind of data? Too vague to be helpful
$flag = true;                           // What is this flag for? Unclear purpose
?>

The Technical Rules PHP Enforces

<?php
// These work perfectly - PHP accepts them
$user_name = "John";           // Letters and underscores - excellent choice
$totalPrice = 99.99;           // CamelCase - also perfectly valid
$_temporary = "temp data";      // Starting underscore - allowed for special cases
$item2 = "second item";        // Numbers after letters - completely fine

// These will break your program - PHP rejects them
$2users = "invalid";           // Cannot start with a number - think about it: how would PHP distinguish this from the number 2 followed by "users"?
$user-name = "broken";         // Hyphens confused with minus signs - PHP thinks you're trying to subtract
$for = "problem";              // "for" is a reserved word - PHP needs it for loops
?>

Variable Scope

A. Global Scope

Variables declared outside functions have global scope:

$globalVar = "I'm global";

function testGlobal() {
    global $globalVar;  // Access global variable
    echo $globalVar;
}

// Alternative using $GLOBALS
function testGlobals() {
    echo $GLOBALS['globalVar'];
}

B. Local Scope

Variables declared inside functions are local:

function testLocal() {
    $localVar = "I'm local";
    echo $localVar;  // Works fine
}

testLocal();
// echo $localVar;  // Error: Undefined variable

C. Static Variables

Retain their value between function calls:

function counter() {
    static $count = 0;
    $count++;
    echo "Count: $count\n";
}

counter(); // Count: 1
counter(); // Count: 2
counter(); // Count: 3

D. Superglobal Variables

Built-in variables available everywhere:

// $_GET, $_POST, $_SESSION, $_COOKIE, $_SERVER, $_ENV, $_FILES
echo $_SERVER['HTTP_HOST'];    // Current domain
echo $_GET['param'];           // URL parameter
echo $_POST['field'];          // Form field

// $GLOBALS contains all global variables
$GLOBALS['customVar'] = "Available everywhere";

Variable References

A. Reference Assignment

$original = "Hello";
$reference = &$original;  // $reference points to $original

$reference = "World";
echo $original;  // Outputs: "World"

// Both variables point to the same memory location
unset($reference);
echo $original;  // Still outputs: "World"

B. Passing by Reference

function modifyValue(&$value) {
    $value = $value * 2;
}

$number = 10;
modifyValue($number);
echo $number;  // Outputs: 20

C. References with Arrays

$array = [1, 2, 3, 4, 5];

// Modify array elements by reference
foreach ($array as &$value) {
    $value *= 2;
}
unset($value); // Always unset reference after foreach

print_r($array); // [2, 4, 6, 8, 10]

Variable Best Practices

Naming Conventions

// Use descriptive names
$userName = "john_doe";           // Good
$u = "john_doe";                  // Bad

// Use camelCase or snake_case consistently
$firstName = "John";              // camelCase
$first_name = "John";             // snake_case

// Boolean variables should be descriptive
$isActive = true;                 // Good
$active = true;                   // Less clear

// Constants in UPPER_CASE
const MAX_RETRY_ATTEMPTS = 3;

Type Safety

// Use strict types (PHP 7+)
declare(strict_types=1);

function addNumbers(int $a, int $b): int {
    return $a + $b;
}

// Input validation
function processAge($age) {
    if (!is_numeric($age) || $age < 0 || $age > 150) {
        throw new InvalidArgumentException('Invalid age provided');
    }
    return (int)$age;
}

Variable Initialization

// Initialize variables before use
$total = 0;
$items = [];
$user = null;

// Use null coalescing operator (PHP 7+)
$username = $_GET['user'] ?? 'guest';
$config = $userConfig ?? $defaultConfig ?? [];

// Null coalescing assignment (PHP 7.4+)
$cache ??= expensive_operation();

Memory Management

// Unset large variables when done
$largeArray = range(1, 1000000);
// ... process array
unset($largeArray);

// Use references for large data structures
function processLargeArray(array &$data) {
    // Process without copying the array
    foreach ($data as &$item) {
        $item = strtoupper($item);
    }
}