Checkpoint: PHP Basics

Reviewed & published by Brayan K

You've learned data types, strings, numbers, type juggling, constants, superglobals and null operators — let's combine them. This checkpoint is one build that exercises everything from the section at once, plus a quiz to confirm it all stuck.

Part of the free PHP course at LearnCodingFast — hands-on lessons with worked examples and the output they print, plus practice exercises and a quick quiz.

What You've Learned

Eight types; the everyday five (string, int, float, bool, null); the falsy seven; get_debug_type.

Interpolation, trim, str_contains, substr, explode/implode, mb_*.

%, **, round/ceil/floor, number_format, the float trap.

Implicit juggling vs explicit casts, == vs ===, strict_types, filter_var.

const vs define, array & class constants, magic constants, global scope.

$_GET/$_POST/$_SERVER, validate-then-escape, ??, ??=, ?->.

The Build: Order Receipt Generator

Here's the brief. You're handed form-style input where every value is a string and one field (coupon) is missing entirely. Your job is to read it safely, validate the numbers, do the money math with a constant tax rate, and print a clean receipt. This single script touches every topic from PHP Basics. Start from the starter below; only reveal the solution once you've genuinely attempted it.

Compare this against your own attempt. The key moves: ?? defaults every read, filter_var validates the numeric strings (returning false on failure, which is why we test with === false), constants hold the tax rate and currency, and number_format formats the money for display only.

<?php
declare(strict_types=1);

$_POST = [
    'name'  => '  ada lovelace  ',
    'qty'   => '3',
    'price' => '19.99',
];

const TAX_RATE = 0.20;
const CURRENCY = '$';

// 1. NAME — default, trim, title-case.
$name = ucwords(trim($_POST['name'] ?? 'guest'));

// 2. QTY — validate, fall back to 1.
$qty = filter_var($_POST['qty'] ?? '', FILTER_VALIDATE_INT);
if ($qty === false) { $qty = 1; }

// 3. PRICE — validate, fall back to 0.0.
$price = filter_var($_POST['price'] ?? '', FILTER_VALIDATE_FLOAT);
if ($price === false) { $price = 0.0; }

// 4. COUPON — null means no discount.
$coupon = $_POST['coupon'] ?? null;
$discount = $coupon === null ? 0.0 : 5.0;

// 5. MATH
$subtotal = $qty * $price;
$subtotal -= $discount;
$tax = $subtotal * TAX_RATE;
$total = $subtotal + $tax;

// 6. OUTPUT
echo "Customer: $name\n";
echo "Items:    $qty x " . CURRENCY . number_format($price, 2) . "\n";
echo "Subtotal: " . CURRENCY . number_format($subtotal, 2) . "\n";
echo "Tax 20%:  " . CURRENCY . number_format($tax, 2) . "\n";
echo "Total:    " . CURRENCY . number_format($total, 2) . "\n";
?>

📝 Checkpoint Quiz

Six questions spanning the whole section. Predict each answer first, then reveal it. If any surprise you, revisit the linked lesson — that's exactly what checkpoints are for.

float(5). The / operator always returns a float, even for whole results. Use intdiv(10, 2) if you need int(5). (See Numbers.)

Only "0.0" is truthy — it's a non-empty string that isn't exactly "0". The other three are all in the falsy seven. (See Data Types.)

?? prints 0 (it only replaces null). ?: would print none because 0 is falsy. (See Null Operators.)

It throws a TypeError — strict types refuse to silently juggle the string into an int. (See Type Juggling.)

Because it's untrusted user input; echoing it raw enables XSS. Wrap output in htmlspecialchars() so injected markup renders as harmless text. (See Superglobals.)

Yes to reading — constants are global by nature. No to reassigning — a constant is write-once; redefining it is a fatal error. (See Constants.)

Pro Tips for the Build

• 💡 Validate, then cast, then calculate. Never do arithmetic on raw string input — filter_var first.
• 💡 Test with === false after filter_var, since a valid 0 is falsy but not failure.
• 💡 Format money only at the end. Keep raw numbers for the math; apply number_format when printing.

Stuck on the Build?

Receipt totals are off by a few cents? Remember floats aren't exact — for a learning exercise number_format(..., 2) rounds the display cleanly; in production you'd work in integer cents.

"Undefined array key 'coupon'"? You read it without ??. Use $_POST['coupon'] ?? null.

Name shows lower-case? Chain the calls in the right order: ucwords(trim(...)) — trim first, then title-case.

🎉 Checkpoint Complete!

• ✅ You combined types, strings, numbers, casting, constants, superglobals and null operators in one script
• ✅ You validated untrusted input before trusting it
• ✅ You used constants and number_format for clean money output
• ✅ You answered the quiz and spotted any gaps to revisit

Practice quiz

Continue this course

• Previous: Null & Null-Safe Operators
• Next: Operators & Expressions — Arithmetic, comparison, logical, and string operators in PHP
• Quick reference: PHP cheat sheet

Related lessons

• Data Types
• Strings
• Numbers & Math
• Type Juggling
• Constants
• Superglobals