PHP — Variables

The $ prefix

Every variable in PHP starts with a $ sign:

<?php
$name = "Alice";      // string
$age = 30;            // integer
$price = 19.99;       // float
$isActive = true;     // boolean

echo $name;           // Alice
echo $age;            // 30
?>

PHP determines the type at runtime based on the value assigned. You do not declare types explicitly.

Naming rules

• Must start with a letter or underscore _
• Can only contain letters, numbers, and underscores
• Are case-sensitive ($name ≠ $Name ≠ $NAME)
• Cannot be a reserved keyword (e.g., $class, $function are valid though — they shadow keywords when prefixed with $)

<?php
$valid_name = "OK";
$valid2 = "OK";
$_private = "OK";
$123 = "INVALID";   // Error: cannot start with a number
$my-name = "INVALID"; // Error: hyphens not allowed
?>

Dynamic typing

PHP is dynamically typed — a variable can change its type at any time:

<?php
$x = 10;        // $x is an integer
$x = "ten";     // Now $x is a string
$x = 10.5;      // Now $x is a float
$x = true;      // Now $x is a boolean
echo gettype($x); // boolean
?>

Type checking

Use gettype() or the var_dump() function to inspect a variable's type:

<?php
$var = 3.14;
echo gettype($var);   // double (float)
var_dump($var);       // float(3.14)
?>

var_dump() outputs both the type and value — useful for debugging:

<?php
$name = "Alice";
$age = 30;
$price = 19.99;

var_dump($name);  // string(5) "Alice"
var_dump($age);   // int(30)
var_dump($price); // float(19.99)
?>

Scope

Variables have different visibility depending on where they are declared:

<?php
// Global scope
$globalVar = "I am global";

function myFunction() {
    // Local scope — cannot access $globalVar here
    $localVar = "I am local";
    echo $localVar;  // OK
    echo $globalVar; // Notice: Undefined variable
}
?>

Use the global keyword to access global variables inside functions:

<?php
$counter = 0;

function increment() {
    global $counter;
    $counter++;
}

increment();
echo $counter; // 1
?>

Alternatively, use the $GLOBALS superglobal array:

<?php
$counter = 0;

function increment() {
    $GLOBALS['counter']++;
}

increment();
echo $GLOBALS['counter']; // 1
?>

Variable variables

PHP supports variable variables — using the value of one variable as the name of another:

<?php
$varName = "greeting";
$$varName = "Hello, world!"; // Creates $greeting

echo $greeting; // Hello, world!
echo $$varName; // Hello, world!
?>

Use variable variables sparingly — they make code harder to follow and debug.

Reference assignment

PHP uses copy-on-assignment by default. To share the same value, use &:

<?php
$a = 10;
$b = $a;  // $b gets a copy of $a's value
$b = 20;
echo $a;  // 10 (unchanged)

$c = &$a; // $c references $a
$c = 20;
echo $a;  // 20 (changed!)
?>

Superglobal variables

PHP provides built-in superglobal arrays available anywhere:

<?php
$_GET    // URL parameters
$_POST   // Form data submitted via POST
$_REQUEST // Combined GET, POST, and COOKIE
$_SERVER  // Server and execution environment info
$_ENV     // Environment variables
$_SESSION // Session variables
$_FILES   // File upload information
$_COOKIE  // Cookie data
$GLOBALS  // All global variables
?>

<?php
// Example: accessing form data
if ($_SERVER["REQUEST_METHOD"] === "POST") {
    $username = $_POST["username"];
    echo "Welcome, $username!";
}
?>

isset() and unset()

Check if a variable exists with isset(), and remove it with unset():

<?php
$var = "Hello";

echo isset($var); // 1 (true)
unset($var);
echo isset($var); // (empty — false)
var_dump($var);   // NULL
?>

Summary

Concept	Example
Declaration	$name = "Alice";
Type check	gettype($var) or var_dump($var)
Scope	global $var; or $GLOBALS['var']
Reference	$b = &$a;
Check existence	isset($var)
Remove	unset($var)

Mini Practice

Write PHP code that:

1. Declares variables of at least 4 different types
2. Uses var_dump() to print each variable's type and value
3. Creates a function that modifies a global variable using the global keyword
4. Demonstrates reference assignment between two variables

Up Next

In the next lesson, you'll learn about Data Types — the different types of values PHP can handle.