PHP Variables

What are Variables?

Variables store data that can be used throughout your script. In PHP, variables start with the $ symbol followed by the variable name. You don’t need to declare a type; PHP figures it out from the value you assign.

$message = "Hello, World!";
$count = 42;
$price = 19.99;
$isActive = true;
$items = ["apple", "banana", "orange"];

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Naming Rules

PHP enforces these rules for variable names:

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

// Valid names
$userName = "john";
$user_name = "john";
$_temporary = "temp";
$item2 = "second";

// Invalid names
// $2users = "invalid";    // Cannot start with number
// $user-name = "broken";  // Hyphens not allowed
// $user name = "broken";  // Spaces not allowed

Descriptive Names

Good variable names make your code easier to understand:

// Clear and descriptive
$customer_email = "alice@email.com";
$monthly_salary = 5000;
$is_logged_in = true;

// Vague and confusing
$x = "alice@email.com";
$data = 5000;
$flag = true;

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Variable Scope

Scope determines where a variable can be accessed.

Global Scope

Variables declared outside functions are global. To access them inside a function, use the global keyword or $GLOBALS array:

$siteName = "My Website";

function showSiteName() {
    global $siteName;
    echo $siteName;
}

// Alternative using $GLOBALS
function showSiteNameAlt() {
    echo $GLOBALS['siteName'];
}

Local Scope

Variables declared inside a function only exist within that function:

function greet() {
    $greeting = "Hello";
    echo $greeting;  // Works
}

greet();
// echo $greeting;  // Error: undefined variable

Static Variables

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

Superglobals

Built-in arrays available everywhere in your script:

$_GET['id'];           // URL parameters (?id=123)
$_POST['email'];       // Form data (POST method)
$_SESSION['user'];     // Session data
$_COOKIE['token'];     // Cookie values
$_SERVER['HTTP_HOST']; // Server/request info

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Variable References

By default, assigning one variable to another copies the value. Using & creates a reference where both variables point to the same data.

Reference Assignment

$original = "Hello";
$reference = &$original;

$reference = "World";
echo $original;  // "World" (both point to same data)

Passing by Reference

Functions can modify the original variable when you pass by reference:

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

$number = 10;
double($number);
echo $number;  // 20

References in Loops

$prices = [10, 20, 30];

foreach ($prices as &$price) {
    $price *= 1.1;  // Add 10% to each
}
unset($price);  // Always unset after reference loop

print_r($prices);  // [11, 22, 33]

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Best Practices

Initialize Variables

Always initialize variables before using them:

$total = 0;
$items = [];
$user = null;

Use Null Coalescing

The ?? operator provides a default when a variable is null or undefined:

$username = $_GET['user'] ?? 'guest';
$config = $customConfig ?? $defaultConfig ?? [];