🐘 PHP Syntax and Variables

What Is PHP?

PHP (a recursive acronym for "PHP: Hypertext Preprocessor") is a server-side scripting language designed for the web. When a browser requests a .php page, the server runs the PHP code, and only the resulting HTML travels to the browser — the source is never exposed.

💡 Analogy: Think of PHP as a chef working behind a kitchen door. The diner (browser) only ever sees the finished plate (HTML). Whatever recipes, measurements, and steps happened in the kitchen (your PHP logic) stay private.

This lesson targets modern PHP (8.0+). The language has evolved dramatically — today it has typed properties, named arguments, match expressions, and strong performance. We'll flag anything version-specific as we go.

Basic Syntax: Tags, Statements, Comments

PHP code lives inside PHP tags. Everything between <?php and ?> is executed; everything outside is passed straight through as plain output (usually HTML).

<?php
echo "Hello from PHP!";
?>

In a file that is pure PHP (no trailing HTML), the convention is to omit the closing ?>. This avoids accidental whitespace being sent to the browser, which can break headers and cookies.

<?php
// A pure-PHP file: note there is no closing tag.
$greeting = "Hello, world";
echo $greeting;

Statements and the semicolon

Every PHP statement ends with a semicolon (;). Forgetting it is the single most common beginner error and produces a "syntax error, unexpected..." message.

<?php
$a = 5;
$b = 10;
echo $a + $b;   // 15

Comments

Comments document your intent and are ignored by the interpreter. PHP supports three styles:

<?php
// Single-line comment (C++ style)

# Single-line comment (shell style)

/*
   Multi-line comment.
   Handy for longer explanations
   or temporarily disabling a block.
*/

Variables

A variable is a named container for a value. In PHP, every variable name begins with a dollar sign ($), followed by a letter or underscore, then any mix of letters, numbers, and underscores.

<?php
$username   = "alice";     // string
$age        = 30;          // integer
$price      = 19.99;       // float
$isActive   = true;        // boolean
$_internal  = "ok";        // leading underscore is allowed
// $2fast    = "no";       // INVALID — cannot start with a digit

You don't declare a type. Assigning a value both creates the variable and gives it a type. Reassigning a different kind of value simply changes the type — this is what "dynamically typed" means.

<?php
$data = "hello";   // $data is now a string
$data = 42;        // the SAME variable is now an integer
$data = [1, 2, 3]; // ...and now it's an array

Naming conventions

PHP itself doesn't enforce a style, but the community standard (PSR) uses camelCase for variables: $firstName, $totalPrice, $isLoggedIn. Choose descriptive names — $userEmail beats $e every time.

Data Types

PHP has a handful of core "scalar" types plus a few compound ones. Here are the ones you'll use daily:

You can inspect a variable's type at runtime with gettype(), or dump its full value and type with var_dump() — an indispensable debugging tool.

<?php
$count = 7;
var_dump($count);        // int(7)
var_dump("7");           // string(1) "7"
var_dump(7 == "7");      // bool(true)  — loose comparison
var_dump(7 === "7");     // bool(false) — strict: type must match too

Type juggling

PHP will automatically convert ("juggle") types in many contexts. For example, using a numeric string in arithmetic converts it to a number:

<?php
$result = "10 apples" + 5;   // 15 in old PHP; a TypeError-ish warning in PHP 8
$clean  = (int) "10";        // explicit cast → int(10)
$asFloat = (float) "3.5kg";  // 3.5

Explicit casts — (int), (float), (string), (bool), (array) — make your intent clear and are far safer than relying on automatic juggling.

Strings & Interpolation

Strings are everywhere in web work — HTML, messages, SQL, JSON. PHP gives you two quote styles that behave differently.

Single vs double quotes

Double quotes parse variables and escape sequences. Single quotes are literal — they treat everything as-is (except \' and \\).

<?php
$name = "Alice";

echo "Hello, $name!\n";   // Hello, Alice! (with a newline)
echo 'Hello, $name!\n';   // Hello, $name!\n  (literal — no parsing)

For clarity when a variable sits next to other characters, wrap it in curly braces:

<?php
$item = "book";
echo "I bought two {$item}s.";   // I bought two books.

Concatenation

The dot (.) operator joins strings together:

<?php
$first = "Ada";
$last  = "Lovelace";
$full  = $first . " " . $last;   // "Ada Lovelace"

$full .= "!";                     // append — now "Ada Lovelace!"

Useful string functions

Constants

A constant is a value that never changes once defined. Unlike variables, constants have no $ and are conventionally written in UPPER_SNAKE_CASE.

<?php
// Modern, preferred form:
const TAX_RATE = 0.08;
const SITE_NAME = "Ray's Shop";

// Older function form (still valid, needed for dynamic names):
define("MAX_UPLOAD_MB", 25);

echo SITE_NAME;              // Ray's Shop
echo 100 * TAX_RATE;        // 8

PHP also ships magic constants that reflect where they're used: __LINE__, __FILE__, __FUNCTION__, and __DIR__ are common in logging and file includes.

Operators

Operators combine or compare values. Here are the families you'll use constantly.

Arithmetic & assignment

<?php
$sum   = 8 + 3;    // 11
$diff  = 8 - 3;    // 5
$prod  = 8 * 3;    // 24
$quot  = 8 / 3;    // 2.6666...
$rem   = 8 % 3;    // 2   (modulo — the remainder)
$pow   = 2 ** 10;  // 1024 (exponent)

$total = 100;
$total += 20;      // shorthand for $total = $total + 20  → 120
$total -= 5;       // 115

Comparison & logical

Handy null-related operators

<?php
// Null coalescing: use the right side if the left is null/unset
$username = $_GET['user'] ?? 'Guest';

// Null coalescing assignment (PHP 7.4+)
$config['timeout'] ??= 30;   // set only if not already set

// Ternary: a compact if/else expression
$label = $isActive ? 'Active' : 'Inactive';

Hands-on Exercise

<?php
const TAX_RATE = 0.08;

$item  = "Mechanical Keyboard";
$price = 79.50;
$qty   = 2;

$subtotal = $price * $qty;
$tax      = $subtotal * TAX_RATE;
$total    = $subtotal + $tax;

echo "Receipt\n";
echo "-------\n";
echo "{$qty} x {$item} @ \${$price}\n";
echo "Subtotal: \$" . number_format($subtotal, 2) . "\n";
echo "Tax (8%): \$" . number_format($tax, 2) . "\n";
echo "TOTAL:    \$" . number_format($total, 2) . "\n";

/*
Receipt
-------
2 x Mechanical Keyboard @ $79.5
Subtotal: $159.00
Tax (8%): $12.72
TOTAL:    $171.72
*/

Summary & Quiz

📚 Further Reading

• PHP Manual — Types
• PHP Manual — Variables
• PHP: The Right Way
• PSR-12 — Coding Style Guide

🚀 What's Next?

Now that you can hold and manipulate data, the next lesson — Control Structures and Functions — shows how to make decisions, repeat work with loops, and package logic into reusable functions.