Prepared statements and why they matter in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a prepared statement execution

What will be the output of this PHP code using a prepared statement?

PHP

<?php
$mysqli = new mysqli('localhost', 'user', 'pass', 'db');
$stmt = $mysqli->prepare('SELECT name FROM users WHERE id = ?');
$id = 2;
$stmt->bind_param('i', $id);
$stmt->execute();
$stmt->bind_result($name);
$stmt->fetch();
echo $name;
?>

A An error because bind_param is missing a parameter

B The name of the user with id 2

C Empty output because fetch() was not called

D The string 'id' printed instead of the name

💡 Hint

Remember that bind_param binds the variable by reference and fetch() retrieves the result.

The prepared statement selects the name where id equals 2. bind_param binds the integer 2 to the placeholder. execute() runs the query. bind_result binds the result to $name. fetch() fetches the row, so echo $name prints the user's name.

🧠 Conceptual

intermediate

Why use prepared statements?

Which of the following is the main reason to use prepared statements in PHP when working with databases?

A They prevent SQL injection by separating code from data

B They allow queries to run without a database connection

C They automatically cache query results for faster access

D They format the output of queries into JSON automatically

💡 Hint

Think about security and how user input is handled.

Prepared statements separate the SQL code from the data values, which prevents attackers from injecting malicious SQL code. This is the main security benefit.

🔧 Debug

advanced

Identify the error in this prepared statement code

What error will this PHP code produce when executed?

PHP

<?php
$mysqli = new mysqli('localhost', 'user', 'pass', 'db');
$stmt = $mysqli->prepare('INSERT INTO users (name, age) VALUES (?, ?)');
$name = 'Alice';
$age = 'twenty';
$stmt->bind_param('si', $name, $age);
$stmt->execute();
?>

A TypeError because 'twenty' is not an integer

B No error, the data is inserted successfully

C SyntaxError due to wrong SQL syntax

D Runtime error because bind_param expects variables passed by reference

💡 Hint

Check the types expected by bind_param and the actual variable types.

bind_param expects the second parameter to be an integer ('i'), but 'twenty' is a string that cannot be converted to integer, causing a TypeError or failure during execution.

📝 Syntax

advanced

Which prepared statement code is syntactically correct?

Which option shows a syntactically correct way to prepare and execute a statement in PHP?

A

$stmt = $mysqli->prepare('SELECT * FROM users WHERE id = ?')
$stmt->bind_param('i', $id)
$stmt->execute()

B

$stmt = $mysqli->prepare('SELECT * FROM users WHERE id = ?');
$stmt->bind_param('i' $id);
$stmt->execute();

C

$stmt = $mysqli->prepare('SELECT * FROM users WHERE id = ?');
$stmt->bind_param(i, $id);
$stmt->execute();

D

$stmt = $mysqli->prepare('SELECT * FROM users WHERE id = ?');
$stmt->bind_param('i', $id);
$stmt->execute();

💡 Hint

Check for missing semicolons and correct quotes around types.

Option D has correct syntax with semicolons and quotes around 'i'. Option D misses semicolons. Option D misses quotes around 'i'. Option D misses a comma between parameters.

🚀 Application

expert

How many rows will be returned?

Given this PHP code using prepared statements, how many rows will be returned by the query?

PHP

<?php
$mysqli = new mysqli('localhost', 'user', 'pass', 'db');
$stmt = $mysqli->prepare('SELECT * FROM users WHERE age > ?');
$age_limit = 30;
$stmt->bind_param('i', $age_limit);
$stmt->execute();
$result = $stmt->get_result();
echo $result->num_rows;
?>

A An error because get_result() is not supported

B All users regardless of age

C Zero, because $age_limit is not defined with $

D The number of users older than 30

💡 Hint

Check variable naming carefully.

The variable $age_limit is missing the $ sign in assignment, so PHP treats it as a constant or undefined variable, causing a warning and $age_limit being null or zero, resulting in zero rows or error.

Practice

1. What is the main reason to use prepared statements in PHP when working with databases?

easy

A. To protect the database from SQL injection attacks

B. To make the code run faster by skipping queries

C. To store data permanently in variables

D. To automatically create database tables

Solution

1. Step 1: Understand the purpose of prepared statements

   Prepared statements separate SQL code from user data, preventing malicious input from changing the query.

2. Step 2: Identify the security benefit

   This separation protects against SQL injection, a common hacking method.

3. Final Answer:

   To protect the database from SQL injection attacks -> Option A

4. Quick Check:

   Security = Protect from SQL injection [OK]

Hint: Prepared statements stop hackers from changing your queries [OK]

Common Mistakes:

• Thinking prepared statements speed up queries significantly
• Confusing prepared statements with data storage
• Believing prepared statements create tables automatically

2. Which of the following is the correct syntax to prepare a statement using PDO in PHP?

easy

A. $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?');

B. $stmt = $pdo->query('SELECT * FROM users WHERE id = ?');

C. $stmt = $pdo->execute('SELECT * FROM users WHERE id = ?');

D. $stmt = $pdo->fetch('SELECT * FROM users WHERE id = ?');

Solution

1. Step 1: Recall PDO method for preparing statements

   The correct method to prepare a statement is prepare(), which sets up the SQL with placeholders.

2. Step 2: Check the options for correct method usage

   Only $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?'); uses prepare() correctly with a SQL string containing a placeholder.

3. Final Answer:

   $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?'); -> Option A

4. Quick Check:

   PDO prepare() method = $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?'); [OK]

Hint: Use prepare() to set up SQL with placeholders [OK]

Common Mistakes:

• Using query() instead of prepare() for statements with placeholders
• Trying to execute SQL directly with execute() without preparing
• Confusing fetch() with prepare()

3. What will be the output of the following PHP code snippet?

$pdo = new PDO('sqlite::memory:');
$pdo->exec('CREATE TABLE users (id INTEGER, name TEXT)');
$stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (?, ?)');
$stmt->execute([1, 'Alice']);
$stmt->execute([2, 'Bob']);
$stmt = $pdo->prepare('SELECT name FROM users WHERE id = ?');
$stmt->execute([2]);
echo $stmt->fetchColumn();

medium

A. 1

B. Alice

C. Bob

D. Error

Solution

1. Step 1: Insert two users into the table

   The code inserts two rows: id=1 with name 'Alice' and id=2 with name 'Bob'.

2. Step 2: Select the name where id = 2

   The prepared statement selects the name for id=2, which is 'Bob'.

3. Final Answer:

   Bob -> Option C

4. Quick Check:

   Selected name for id=2 = Bob [OK]

Hint: Check which id is queried to find the correct name [OK]

Common Mistakes:

• Confusing the order of inserted rows
• Expecting the first inserted name instead of the queried one
• Assuming an error due to missing database file

4. Identify the error in the following PHP code using prepared statements:

$stmt = $pdo->prepare('SELECT * FROM users WHERE email = :email');
$stmt->execute('[e-mail]');
$result = $stmt->fetchAll();

medium

A. The SQL query is missing a WHERE clause

B. The prepare() method is missing a placeholder

C. fetchAll() cannot be used after execute()

D. The execute() method should receive an array, not a string

Solution

1. Step 1: Check the execute() method argument

   When using named placeholders like ':email', execute() expects an associative array with keys matching placeholders.

2. Step 2: Identify the incorrect argument type

   The code passes a string directly instead of an array like ['email' => '[e-mail]'].

3. Final Answer:

   The execute() method should receive an array, not a string -> Option D

4. Quick Check:

   execute() needs array for named placeholders [OK]

Hint: Pass an array to execute() when using named placeholders [OK]

Common Mistakes:

• Passing a string instead of an array to execute()
• Forgetting to match placeholder names in the array keys
• Thinking fetchAll() cannot follow execute()

5. You want to insert multiple user records safely using prepared statements in PHP. Which approach below correctly uses prepared statements to insert two users with ids and names?

hard

A. $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (?, ?)');
$stmt->execute(1, 'Alice');
$stmt->execute(2, 'Bob');

B. $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)');
$stmt->execute(['id' => 1, 'name' => 'Alice']);
$stmt->execute(['id' => 2, 'name' => 'Bob']);

C. $pdo->exec("INSERT INTO users (id, name) VALUES (1, 'Alice'), (2, 'Bob')");

D. $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)');
$stmt->execute(['id' => 1, 'name' => 'Alice', 'id' => 2, 'name' => 'Bob']);

Solution

1. Step 1: Understand how to use named placeholders for multiple inserts

   Using named placeholders (:id, :name) allows binding values clearly for each execute call.

2. Step 2: Check each option for correct usage

   Only $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)'); $stmt->execute(['id' => 1, 'name' => 'Alice']); $stmt->execute(['id' => 2, 'name' => 'Bob']); correctly prepares once and executes twice with separate associative arrays for each user.

3. Step 3: Identify errors in other options

   $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (?, ?)'); $stmt->execute(1, 'Alice'); $stmt->execute(2, 'Bob'); incorrectly passes parameters directly to execute() without an array; $pdo->exec("INSERT INTO users (id, name) VALUES (1, 'Alice'), (2, 'Bob')"); uses exec() without preparing; $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)'); $stmt->execute(['id' => 1, 'name' => 'Alice', 'id' => 2, 'name' => 'Bob']); tries to pass multiple users in one execute call incorrectly.

4. Final Answer:

   $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)'); $stmt->execute(['id' => 1, 'name' => 'Alice']); $stmt->execute(['id' => 2, 'name' => 'Bob']); -> Option B

5. Quick Check:

   Named placeholders + separate execute calls = $stmt = $pdo->prepare('INSERT INTO users (id, name) VALUES (:id, :name)'); $stmt->execute(['id' => 1, 'name' => 'Alice']); $stmt->execute(['id' => 2, 'name' => 'Bob']); [OK]

Hint: Use named placeholders and separate execute() calls per record [OK]

Common Mistakes:

• Passing multiple records in one execute call incorrectly
• Not using prepared statements and risking SQL injection
• Passing parameters to execute() without an array for positional placeholders