Preventing injection with prepared statements in PHP

Introduction

Prepared statements help keep your database safe by stopping bad code from sneaking in. They make sure user input is treated as data, not code.

When you get user input like names or emails to save in a database.

When you want to search or filter data based on what a user types.

When you build login forms that check usernames and passwords.

When you update or delete database records using user input.

Whenever you want to avoid security problems like SQL injection attacks.

Syntax

PHP

<?php
$stmt = $pdo->prepare('SQL query with ? or :named placeholders');
$stmt->execute([$value1, $value2]);
// or
$stmt->execute(['name' => $value1, 'age' => $value2]);

Use prepare() to create a safe SQL template.

Use execute() to fill in the placeholders with actual values.

Examples

Using question mark ? as a placeholder for one value.

PHP

<?php
$stmt = $pdo->prepare('SELECT * FROM users WHERE email = ?');
$stmt->execute([$email]);

Using named placeholder :user for clarity.

PHP

<?php
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :user');
$stmt->execute(['user' => $username]);

Inserting data safely with named placeholders.

PHP

<?php
$stmt = $pdo->prepare('INSERT INTO products (name, price) VALUES (:name, :price)');
$stmt->execute(['name' => $productName, 'price' => $productPrice]);

Sample Program

This program shows how to safely add and get a user from a database using prepared statements. It stops any harmful input from changing the database commands.

PHP

<?php
// Connect to database using PDO
$pdo = new PDO('sqlite::memory:');
// Create a simple table
$pdo->exec('CREATE TABLE users (id INTEGER PRIMARY KEY, username TEXT, email TEXT)');

// User input simulation
$username = "alice";
$email = "[e-mail]";

// Prepare statement to insert data safely
$stmt = $pdo->prepare('INSERT INTO users (username, email) VALUES (:username, :email)');
$stmt->execute(['username' => $username, 'email' => $email]);

// Prepare statement to select data safely
$stmt = $pdo->prepare('SELECT id, username, email FROM users WHERE username = :username');
$stmt->execute(['username' => $username]);

// Fetch and print result
$user = $stmt->fetch(PDO::FETCH_ASSOC);
if ($user) {
    echo "User found: " . $user['username'] . " with email " . $user['email'] . "\n";
} else {
    echo "User not found.\n";
}

Important Notes

Always use prepared statements when working with user input to avoid SQL injection.

Placeholders can be question marks ? or named like :name. Named placeholders are easier to read.

Prepared statements also help your code run faster if you repeat the same query many times with different values.

Summary

Prepared statements keep your database safe by separating code from data.

Use prepare() and execute() to run safe queries.

Always use them when you include user input in SQL commands.

Practice

1. What is the main purpose of using prepared statements in PHP when working with databases?

easy

A. To allow multiple database connections at once

B. To prevent SQL injection by separating SQL code from user data

C. To automatically format the output of SQL queries

D. To speed up the execution of all SQL queries

Solution

1. Step 1: Understand SQL injection risks

   SQL injection happens when user input is mixed directly with SQL code, allowing attackers to change the query.

2. Step 2: Role of prepared statements

   Prepared statements separate the SQL code from the data, so user input cannot change the query structure.

3. Final Answer:

   To prevent SQL injection by separating SQL code from user data -> Option B

4. Quick Check:

   Prepared statements prevent injection = D [OK]

Hint: Prepared statements separate code and data to stop injection [OK]

Common Mistakes:

• Thinking prepared statements speed up all queries
• Believing they format output automatically
• Confusing with multiple connections

2. Which of the following is the correct way to prepare and execute a statement in PHP PDO to prevent SQL injection?

easy

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

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

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

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

Solution

1. Step 1: Check correct prepare syntax

   Use prepare() with placeholders like ? for parameters.

2. Step 2: Execute with parameters

   Call execute() with an array of values to bind to placeholders.

3. Final Answer:

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

4. Quick Check:

   prepare + execute with array = A [OK]

Hint: Use prepare() then execute() with array for safe queries [OK]

Common Mistakes:

• Using query() with variables directly
• Putting variables inside prepare() string
• Calling execute() before prepare()

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

$stmt = $pdo->prepare('SELECT name FROM users WHERE id = ?');
$stmt->execute([2]);
$result = $stmt->fetchColumn();
echo $result;

Assuming the user with id 2 has the name 'Alice'.

medium

A. 2

B. Array

C. Alice

D. Error

Solution

1. Step 1: Understand prepare and execute

   The statement prepares a query with a placeholder and executes it with id = 2.

2. Step 2: Fetch the result

   fetchColumn() returns the first column of the first row, which is the name 'Alice'.

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   fetchColumn returns name 'Alice' = C [OK]

Hint: fetchColumn() returns first column value from executed statement [OK]

Common Mistakes:

• Expecting fetchColumn() to return an array
• Confusing id with name output
• Not passing parameters to execute()

4. Identify the error in this PHP code that uses prepared statements:

$stmt = $pdo->prepare('SELECT * FROM users WHERE email = :email');
$stmt->execute(':email' => $email);

medium

A. execute() should be called before prepare()

B. The placeholder should be a question mark (?) instead of :email

C. prepare() cannot use named placeholders like :email

D. The execute() method argument should be an array, but syntax is incorrect

Solution

1. Step 1: Check placeholder usage

   Named placeholders like :email are valid in prepare statements.

2. Step 2: Check execute() argument syntax

   execute() expects an array, but ':email' => $email is not inside an array here.

3. Final Answer:

   The execute() method argument should be an array, but syntax is incorrect -> Option D

4. Quick Check:

   execute() needs array argument = A [OK]

Hint: Pass parameters as array to execute() [OK]

Common Mistakes:

• Using wrong placeholder style
• Passing parameters without array brackets
• Calling execute() before prepare()

5. You want to insert multiple user records safely using prepared statements in PHP. Which approach correctly prevents SQL injection and efficiently inserts multiple rows?

$users = [
  ['name' => 'John', 'email' => '[e-mail]'],
  ['name' => 'Jane', 'email' => '[e-mail]']
];
// Which code below is correct?

hard

A. foreach ($users as $user) {
$stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)');
$stmt->execute([$user['name'], $user['email']]);
}

B. $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)');
$stmt->execute($users);

C. foreach ($users as $user) {
$sql = "INSERT INTO users (name, email) VALUES ('{$user['name']}', '{$user['email']}')";
$pdo->query($sql);
}

D. $sql = 'INSERT INTO users (name, email) VALUES (?, ?)';
$stmt = $pdo->prepare($sql);
$stmt->execute();

Solution

1. Step 1: Check safe insertion of multiple rows

   foreach ($users as $user) { $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)'); $stmt->execute([$user['name'], $user['email']]); } prepares once per iteration and executes with user data safely bound as parameters.

2. Step 2: Analyze other options

   $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)'); $stmt->execute($users); passes array of arrays directly to execute(), which is invalid. foreach ($users as $user) { $sql = "INSERT INTO users (name, email) VALUES ('{$user['name']}', '{$user['email']}')"; $pdo->query($sql); } uses string interpolation risking injection. $sql = 'INSERT INTO users (name, email) VALUES (?, ?)'; $stmt = $pdo->prepare($sql); $stmt->execute(); calls execute() without parameters.

3. Final Answer:

   foreach ($users as $user) { $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)'); $stmt->execute([$user['name'], $user['email']]); } -> Option A

4. Quick Check:

   Prepare and execute per user with parameters = B [OK]

Hint: Prepare inside loop and execute with user data array [OK]

Common Mistakes:

• Passing multidimensional array directly to execute()
• Using string interpolation instead of parameters
• Calling execute() without parameters