Variables, Types & Constants — PHP Course

What you'll learn

Programs become useful when they can remember values. You'll learn how PHP names, classifies, inspects, and converts data—and when a value should become a constant.

Create clearly named variables and update their values
Recognize PHP's everyday scalar and compound types
Inspect uncertain data with var_dump() and gettype()
Choose explicit conversion over surprising implicit coercion
Declare values that must not change

Variables are named values

A PHP variable begins with $, followed by a case-sensitive name. Assignment uses =: the expression on the right is evaluated, then stored under the name on the left.

cart.php

<?php

$customerName = "Lina";
$itemCount = 3;
$subtotal = 24.50;
$hasCoupon = true;

echo "{$customerName} has {$itemCount} items.";

Tip

Use camelCase for local variables and choose names that reveal meaning: $subtotal is more useful than $x or $floatValue.

The core PHP types

Type	Examples	Use
`int`	`42, -7`	Whole numbers
`float`	`19.99, 1.5e3`	Decimal and scientific values
`string`	`"PHP"`	Text and bytes
`bool`	`true, false`	Yes/no decisions
`array`	`["a", "b"]`	Ordered maps used as lists or records
`object`	`new DateTime()`	State and behavior packaged as a type
`null`	`null`	Intentional absence of a value

PHP is dynamically typed: a variable can refer to values of different types at different times. The value has a type even though the variable declaration does not spell one out.

Key idea

Dynamic typing removes declaration ceremony; it does not remove the need to reason about types. Treat input, domain values, and output formats as explicit contracts.

Inspect values while developing

var_dump() shows a value's type and detailed structure. gettype() returns a readable type name. They are development tools—don't leave raw dumps in a production response.

inspect.php

<?php

$quantity = 4;
$price = 7.25;
$tags = ["php", "web"];
$discount = null;

var_dump($quantity, $price, $tags, $discount);
echo gettype($tags);

For boolean questions, prefer focused checks such as is_int(), is_string(), is_array(), and is_null().

Casting and type juggling

PHP sometimes coerces compatible values automatically. That convenience can hide bugs at request and persistence boundaries, so convert deliberately when a source gives you text but your domain needs a number or boolean.

convert.php

<?php

$quantityText = "4";
$quantity = (int) $quantityText;
$price = (float) "7.25";
$isPublished = (bool) 1;

$total = $quantity * $price;
echo number_format($total, 2);

Validate before casting

Casting "four" to int does not produce a meaningful quantity. First validate that the input represents an allowed integer; then convert it.

Constants communicate invariants

Use a constant when a name should never be reassigned during execution. const is clear for values known in source code; define() can declare one at runtime.

constants.php

<?php

const TAX_RATE = 0.11;
define('APP_NAME', 'Budget Board');

$subtotal = 100;
$total = $subtotal * (1 + TAX_RATE);

echo APP_NAME . ': ' . $total;

Recap & quick check

Key takeaways

PHP variables start with $ and should be named for their meaning.
Every value has a type even though PHP variables are dynamically typed.
Use var_dump for detailed development inspection and is_* functions for focused checks.
Validate external text before explicitly converting it to a domain type.
Use constants to name values that must not change during a run.

Quick check

1. Which name follows normal PHP variable syntax?

2. Which tool shows both a value's type and structure?

3. What should happen before casting request text to an integer?