Preventing injection with prepared statements in PHP - Time & Space Complexity

Time Complexity: Preventing injection with prepared statements

O(n)

Understanding Time Complexity

We want to understand how the time needed to run prepared statements changes as we handle more data.

How does the program's work grow when using prepared statements to prevent injection?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Prepare the SQL once
$stmt = $pdo->prepare('SELECT * FROM users WHERE email = ?');

// Loop through emails to execute the statement
foreach ($emails as $email) {
    $stmt->execute([$email]);
    $result = $stmt->fetchAll();
}
    

This code prepares a query once and runs it multiple times with different emails to safely get user data.

Identify Repeating Operations

• Primary operation: Executing the prepared statement and fetching results inside the loop.
• How many times: Once for each email in the input list.

How Execution Grows With Input

Each new email means one more execution and fetch operation.

Input Size (n)	Approx. Operations
10	About 10 executions and fetches
100	About 100 executions and fetches
1000	About 1000 executions and fetches

Pattern observation: The work grows directly with the number of emails; doubling emails doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows in a straight line as the number of emails increases.

Common Mistake

[X] Wrong: "Preparing the statement inside the loop is just as fast as preparing it once."

[OK] Correct: Preparing inside the loop repeats setup work each time, making the program slower as input grows.

Interview Connect

Understanding how prepared statements scale helps you write safe and efficient code, a skill valued in real projects and interviews.

Self-Check

"What if we prepared the statement inside the loop instead of once before it? How would the time complexity change?"

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