Prepared statements and why they matter in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a prepared statement in PHP?

A prepared statement is a feature that lets you write a SQL query template with placeholders. You fill in the placeholders later with actual values before running the query. This helps keep your database safe and your code efficient.

beginner

Why do prepared statements help prevent SQL injection?

Prepared statements separate the SQL code from the data. This means user input is treated only as data, not as part of the SQL command. So, attackers cannot change the query by injecting harmful SQL code.

intermediate

How do prepared statements improve performance?

The database server compiles the SQL query once, then runs it many times with different data. This saves time because the server doesn’t have to parse and plan the query each time.

beginner

Show a simple PHP example of using a prepared statement to select a user by ID.

$stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?'); $stmt->execute([$userId]); $user = $stmt->fetch();

beginner

What happens if you don’t use prepared statements and insert user input directly into SQL?

If you insert user input directly, attackers can add SQL code that changes your query. This can lead to data leaks, data loss, or unauthorized access. This is called SQL injection.

What is the main security benefit of using prepared statements?

A They make the website load faster

B They prevent SQL injection attacks

C They allow multiple users to connect at once

D They automatically encrypt data

Prepared statements keep SQL code and data separate, stopping attackers from injecting harmful SQL.

In PHP, which method prepares a SQL statement for execution?

A $pdo->execute()

B $pdo->query()

C $pdo->fetch()

D $pdo->prepare()

The prepare() method creates a prepared statement with placeholders.

What symbol is commonly used as a placeholder in prepared statements?

A #

B &

C ?

D $

The question mark (?) is used as a placeholder for values in prepared statements.

Which of these is NOT a benefit of prepared statements?

A Automatic data encryption

B Improved security

C Better performance

D Cleaner code

Prepared statements do not encrypt data automatically.

What happens if you execute a prepared statement multiple times with different data?

A The query runs faster because it’s compiled once

B The database recompiles the query each time

C The query fails after the first execution

D The data is ignored

Prepared statements compile the query once, so running it multiple times with new data is faster.

Explain in your own words why prepared statements are important for database security.

Think about how attackers try to change SQL queries.

Describe the steps to use a prepared statement in PHP to safely query a database.

Remember the order: prepare, execute, fetch.

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