PHP Variables

PHP Variables: Naming Rules, Types, and Scope

A PHP variable is a named container that holds data such as text, numbers, arrays, or objects in memory. Every PHP variable starts with a dollar sign $ followed by an identifier name, and PHP assigns its type dynamically when you store a value.

PHP Variable Naming Rules

Follow these four rules whenever you name a variable in PHP:

• Start with a letter or underscore: $userAge and $_cacheKey are valid, whereas $2ndPlace causes a syntax error.
• Use only letters, numbers, and underscores: Avoid spaces, hyphens, or symbols like $user-name.
• Keep casing consistent: Variable names are case-sensitive, so $orderId and $orderid are distinct.
• Never reassign $this: $this is reserved inside classes to point to the active object instance.

<?php
declare(strict_types=1);

$customerName = "Nadia Karim";
$itemCount = 3;
$unitPrice = 14.50;
$totalCost = $itemCount * $unitPrice;

echo "{$customerName} ordered {$itemCount} items for $" . number_format($totalCost, 2) . PHP_EOL;

Variable Scope: Local, Global, and Static

Where you declare a variable controls which parts of your script can read or change it. Variables created inside a function stay local to that function and disappear once it finishes.

<?php
$siteCurrency = "USD"; // Global scope

function formatAmount(float $price): string
{
    global $siteCurrency; // Pull global variable into function scope
    static $callCount = 0; // Preserved across function calls
    $callCount++;

    return "[#{$callCount}] {$siteCurrency} " . number_format($price, 2);
}

echo formatAmount(25.0) . PHP_EOL;
echo formatAmount(40.5) . PHP_EOL;

Passing values into functions as parameters is much cleaner than relying on the global keyword, because it keeps dependencies explicit and easy to test.

Common Mistakes to Avoid

• Using undefined variables: Reading $discount before assigning it triggers a warning in PHP 8 and defaults to null.
• Overusing global: Mutating global variables inside functions creates hidden side effects that make debugging painful.
• Accidental assignment inside if: Writing if ($isAdmin = true) assigns true instead of checking equality with ===.

Frequently Asked Questions

Do I need to declare a variable type in PHP?

No. PHP infers the type from the assigned value automatically, though you can enforce strict types on function parameters, return values, and class properties.

How do I check if a variable exists and is not null?

Use isset($var) to confirm a variable is declared and non-null, or use the null coalescing operator $name = $input ?? 'Guest'.

What does a static variable inside a function do?

A static local variable keeps its value between multiple calls to that same function instead of resetting on every call.