PHP Debugging Challenges: Find and Fix the Bug

Bug 1: the missing first item

index.php

PHP

<?php
$items = ["pen", "book", "bag"];

for ($i = 1; $i < count($items); $i++) {
    echo $items[$i] . " ";
}

What is printed, and what was intended?

Show the answer

It prints book bag. The loop starts at 1, but array positions start at 0. Fix: $i = 0, or use foreach ($items as $item).

index.php

PHP

<?php
foreach (["pen", "book", "bag"] as $item) {
    echo $item . " ";
}

Output

OUTPUT

pen book bag

Bug 2: “Not found”

index.php

PHP

<?php
$email = "ana@example.com";

if (strpos($email, "ana") == false) {
    echo "Not found";
} else {
    echo "Found";
}

Show the answer

strpos() returns 0 (the position), and 0 == false is true. Use ===, or str_contains().

index.php

PHP

<?php
$email = "ana@example.com";

echo str_contains($email, "ana") ? "Found" : "Not found";

Output

OUTPUT

Found

Bug 3: the last item changes

index.php

PHP

<?php
$list = [1, 2, 3];

foreach ($list as &$v) {
}

foreach ($list as $v) {
}

echo implode(",", $list);

Show the answer

The output is 1,2,2. After the first loop, $v is still a reference to the last element, so the second loop writes each value into it. Fix: unset($v); after the first loop, or avoid references.

index.php

PHP

<?php
$list = [1, 2, 3];

foreach ($list as &$v) {
}
unset($v);

foreach ($list as $v) {
}

echo implode(",", $list);

Output

OUTPUT

1,2,3

Bug 4: the total is always 0

index.php

PHP

<?php
$total = 0;

function addToTotal(int $amount): void
{
    $total += $amount;
}

addToTotal(5);
addToTotal(10);

echo $total;

Show the answer

The function works on its own local $total (and PHP warns about an undefined variable). Return the new value instead:

index.php

PHP

<?php
function addToTotal(int $total, int $amount): int
{
    return $total + $amount;
}

$total = 0;
$total = addToTotal($total, 5);
$total = addToTotal($total, 10);

echo $total;

Output

OUTPUT

Bug 5: the greeting shows markup

index.php

PHP

<?php
$name = $_GET["name"] ?? "guest";

echo "<h1>Hello, $name</h1>";

What is wrong with it?

Show the answer

XSS. ?name=<script>... runs script in the visitor’s browser. Escape output:

index.php

PHP

<?php
$name = '<script>alert(1)</script>';

echo "<h1>Hello, " . htmlspecialchars($name, ENT_QUOTES, "UTF-8") . "</h1>";

Output

OUTPUT

<h1>Hello, &lt;script&gt;alert(1)&lt;/script&gt;</h1>

Bug 6: the price is wrong

index.php

PHP

<?php
$price = "19.99";
$quantity = "3";

echo $price * $quantity . " " . ($price * $quantity == 59.97 ? "exact" : "not exact");

What does it print, and why is it risky?

Show the answer

It prints 59.97 not exact. Floating-point numbers are approximate, so 19.99 * 3 is not exactly 59.97. Do money calculations with integers (cents) or BCMath, and compare floats with a tolerance.

index.php

PHP

<?php
$cents = 1999 * 3;

echo number_format($cents / 100, 2) . " " . ($cents === 5997 ? "exact" : "not exact");

Output

OUTPUT

59.97 exact

Bug 7: “headers already sent”

index.php

PHP

<?php
echo "Welcome!";
header("Location: /dashboard");
exit;

Show the answer

header() must be called before any output. echo already sent the body. Remove the output (or buffer it with ob_start()), and check for stray whitespace or a byte order mark before <?php in included files.

index.php

PHP

1
2
3

<?php
header("Location: /dashboard");
exit;

Bug 8: users see each other’s data

index.php

PHP

1
2
3

<?php
$id = $_GET["id"];
$order = $pdo->query("SELECT * FROM orders WHERE id = $id")->fetch();

Name two problems.

Show the answer

SQL injection (the id is joined into the SQL).
IDOR (no check that the order belongs to the logged-in user).

index.php

PHP

<?php
$id = filter_var($_GET["id"] ?? null, FILTER_VALIDATE_INT);
$stmt = $pdo->prepare("SELECT id, total FROM orders WHERE id = :id AND user_id = :user");
$stmt->execute(["id" => $id, "user" => $_SESSION["user_id"]]);
$order = $stmt->fetch();

Bug 9: the names are in the wrong order

index.php

PHP

<?php
$names = ["ben", "Cara", "ana", "Dev"];
sort($names);

echo implode(", ", $names);

The list should be alphabetical, ignoring case. What is printed?

Show the answer

It prints Cara, Dev, ana, ben. By default sort() compares strings byte by byte, so every capital letter comes before every small letter. Ask for a case-insensitive sort:

index.php

PHP

<?php
$names = ["ben", "Cara", "ana", "Dev"];

sort($names, SORT_STRING | SORT_FLAG_CASE);

echo implode(", ", $names);

Output

OUTPUT

ana, ben, Cara, Dev

For human-friendly sorting of numbers inside text (file2, file10), use natcasesort() or SORT_NATURAL.

Bug 10: the cart is empty after redirect

index.php

PHP

<?php
$cart = ["pen"];
header("Location: cart.php");
exit;
// cart.php: var_dump($cart);

Show the answer

Variables do not survive a redirect: each request starts fresh. Store the cart in the session (or the database).

index.php

PHP

<?php
session_start();
$_SESSION["cart"] = ["pen"];
header("Location: cart.php");
exit;