Variables & Data Types 📦

Master PHP's dollar-sign variables, data types, type juggling, constants, and superglobals.

What You'll Learn in This Lesson

• Declaring variables with the $ prefix and naming conventions
• PHP data types: strings, integers, floats, booleans, NULL
• String interpolation with double quotes vs single quotes
• Type juggling (automatic) vs explicit type casting
• Constants with define() and const, plus superglobals

Note: PHP runs on a server. Our examples simulate PHP logic using JavaScript. To run real PHP, install PHP and test locally with php -S localhost:8000.

🎯 Real-World Analogy: Think of variables as labelled boxes in a warehouse. The label ($name) tells you what's inside, and you can swap the contents anytime. The $ sign is like a sticker that says "this is a variable" — every box must have one.

1️⃣ PHP Data Types

Type	Example	Check Function
String	"Hello World"	is_string()
Integer	42, -10, 0	is_int()
Float	3.14, -0.5	is_float()
Boolean	true, false	is_bool()
NULL	null	is_null()
Array	[1, 2, 3]	is_array()

Try It: PHP Variables

Declare variables, check types, and see string interpolation in action

Try it Yourself »

JavaScript

// PHP Variables & Data Types (simulated in JavaScript)
// In PHP, every variable starts with $

console.log("=== PHP Variables ===");
console.log();

// String
let name = "Alice Johnson";     // PHP: $name = "Alice Johnson";
console.log("String:  $name = " + JSON.stringify(name));

// Integer
let age = 28;                   // PHP: $age = 28;
console.log("Integer: $age = " + age);

// Float
let price = 49.99;              // PHP: $price = 49.99;
console.log("Float:   $price = " + price);

// Bo
...

2️⃣ Type Juggling & Casting

PHP automatically converts types when mixing them (type juggling). You can also force a conversion with explicit casting:

// Type juggling (automatic)
$result = "5" + 3;    // 8 (string "5" becomes int)
$result = true + 1;   // 2 (true becomes 1)

// Explicit casting
$x = (int)"42.7";     // 42 (truncates decimal)
$y = (string)100;     // "100"
$z = (bool)"";        // false (empty string = false)

⚠️

Watch out: "hello" + 5 = 5 — non-numeric strings become 0!

Try It: Type Casting & Constants

Explore automatic type juggling, explicit casting, constants, and superglobals

Try it Yourself »

JavaScript

// PHP Type Casting & Type Juggling
console.log("=== Type Juggling (Automatic Conversion) ===");
console.log();
console.log("PHP automatically converts types when needed:");
console.log();

// PHP type juggling examples
let strNum = "42";
let num = 8;
console.log('"42" + 8 = ' + (parseInt(strNum) + num));
console.log("  → PHP converts the string '42' to integer 42, then adds");
console.log();

console.log('"hello" + 5 = ' + (0 + 5));
console.log("  → 'hello' becomes 0 (no number found), so 0 + 5
...

⚠️ Common Mistakes

⚠️

Forgetting the $ — name = "Alice" is a constant, not a variable. You need $name.

⚠️

Single vs double quotes — "Hello $name" interpolates, 'Hello $name' outputs the literal text.

⚠️

Comparing with == vs === — 0 == "hello" is true (type juggling!), use === for strict comparison.

💡

Pro Tip: Always use === (strict equality) in PHP to avoid type juggling surprises.

📋 Quick Reference — Variables & Types

Syntax	Example	Notes
$var = value	$age = 25;	Declare variable
gettype()	gettype($age)	Returns "integer"
(int)	(int)"42"	Cast to integer
define()	define('PI', 3.14);	Define constant
===	$a === $b	Strict equality

🎉 Lesson Complete!

You've mastered PHP variables and data types! Next, learn about operators and expressions.