Error handling with PDO exceptions in PHP - Mini Project: Build & Apply

Error handling with PDO exceptions

📖 Scenario: You are building a simple PHP script to connect to a database and fetch user data. Sometimes, the connection or query might fail. To handle these problems safely, you will use PDO exceptions.

🎯 Goal: Create a PHP script that connects to a database using PDO, runs a query to get user names, and handles any errors using PDO exceptions.

📋 What You'll Learn

Create a PDO connection to a MySQL database with given credentials.

Set the PDO error mode to throw exceptions.

Write a try-catch block to catch PDO exceptions.

Run a simple SELECT query to get user names from a table called users.

Print the user names if the query succeeds.

Print an error message if an exception occurs.

💡 Why This Matters

🌍 Real World

Handling database errors gracefully is important in real web applications to avoid crashes and show friendly messages.

💼 Career

Knowing how to use PDO exceptions is a key skill for PHP developers working with databases securely and reliably.

1

Create a PDO connection

Create a new PDO object called $pdo to connect to a MySQL database with these exact details: host localhost, database name testdb, username root, and password password123.

PHP

<?php
// Create a new PDO connection to MySQL
// Your code here

Hint

Use new PDO() with the DSN string and credentials exactly as given.

2

Set PDO error mode to exceptions

Set the PDO attribute PDO::ATTR_ERRMODE on $pdo to PDO::ERRMODE_EXCEPTION using the setAttribute method.

PHP

<?php
$pdo = new PDO('mysql:host=localhost;dbname=testdb', 'root', 'password123');
// Set PDO error mode to exceptions
// Your code here

Hint

Use $pdo->setAttribute() to set the error mode.

3

Write try-catch block with query

Write a try block where you run the query SELECT name FROM users using $pdo->query() and fetch all results with fetchAll(PDO::FETCH_ASSOC) into a variable called $users. Then write a catch block to catch PDOException as $e.

PHP

setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
// Add try-catch block to run query and catch exceptions
// Your code here

Hint

Use try { ... } catch (PDOException $e) { ... } and run the query inside the try block.

4

Print results or error message

Inside the try block, use a foreach loop with variables $user to print each user name with echo $user['name'] . "\n";. Inside the catch block, print the error message with echo "Error: " . $e->getMessage();.

PHP

setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

try {
    $stmt = $pdo->query('SELECT name FROM users');
    $users = $stmt->fetchAll(PDO::FETCH_ASSOC);
    // Print each user name
    // Your code here
} catch (PDOException $e) {
    // Print error message
    // Your code here
}

Hint

Use foreach ($users as $user) to print names and echo "Error: " . $e->getMessage(); in catch.

Practice

1. What is the main purpose of using try and catch blocks when working with PDO exceptions in PHP?

easy

A. To create new database tables

B. To handle database errors without stopping the entire program

C. To automatically fix SQL syntax errors

D. To speed up database queries

Solution

1. Step 1: Understand the role of try-catch

   The try block runs code that might cause an error, and catch handles that error safely.

2. Step 2: Relate to PDO exceptions

   PDO exceptions occur on database errors; using try-catch prevents the program from stopping abruptly.

3. Final Answer:

   To handle database errors without stopping the entire program -> Option B

4. Quick Check:

   try-catch handles errors safely = D [OK]

Hint: Try-catch blocks catch errors to keep programs running [OK]

Common Mistakes:

• Thinking try-catch speeds up queries
• Believing it fixes SQL errors automatically
• Confusing error handling with database creation

2. Which of the following is the correct way to set PDO to throw exceptions on errors?

easy

A. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_SILENT);

B. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING);

C. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

D. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_IGNORE);

Solution

1. Step 1: Identify PDO error modes

   PDO supports error modes: SILENT, WARNING, EXCEPTION, and IGNORE (not standard).

2. Step 2: Choose the mode that throws exceptions

   PDO::ERRMODE_EXCEPTION makes PDO throw exceptions on errors.

3. Final Answer:

   $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); -> Option C

4. Quick Check:

   Exception mode = PDO::ERRMODE_EXCEPTION = B [OK]

Hint: Use ERRMODE_EXCEPTION to get PDO exceptions [OK]

Common Mistakes:

• Using ERRMODE_SILENT which hides errors
• Confusing ERRMODE_WARNING with exceptions
• Using non-existent ERRMODE_IGNORE

3. What will be the output of this PHP code?

$pdo = new PDO('sqlite::memory:');
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
try {
    $pdo->exec('CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT)');
    $pdo->exec('INSERT INTO users (id, name) VALUES (1, "Alice")');
    $pdo->exec('INSERT INTO users (id, name) VALUES (1, "Bob")');
    echo "Insert successful";
} catch (PDOException $e) {
    echo "Error: " . $e->getMessage();
}

medium

A. Insert successful

B. Syntax error in SQL statement

C. No output

D. Error: SQLSTATE[23000]: Integrity constraint violation: UNIQUE constraint failed: users.id

Solution

1. Step 1: Understand the code flow

   The code creates a table with a primary key on 'id' and inserts two rows with the same id=1.

2. Step 2: Identify the error

   Inserting duplicate primary keys causes an integrity constraint violation, triggering a PDOException.

3. Final Answer:

   Error: SQLSTATE[23000]: Integrity constraint violation: UNIQUE constraint failed: users.id -> Option D

4. Quick Check:

   Duplicate primary key causes exception = C [OK]

Hint: Duplicate primary keys cause PDOException errors [OK]

Common Mistakes:

• Expecting 'Insert successful' despite duplicate keys
• Thinking it's a syntax error
• Ignoring exception handling output

4. Identify the error in this PHP code snippet that uses PDO exceptions:

try {
    $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'user', 'pass');
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $pdo->exec('INVALID SQL');
} catch (PDOException $e) {
    echo 'Error: ' . $e->getMessage();
}

medium

A. SQL statement is invalid causing exception

B. Error mode is not set to exceptions

C. PDO connection string is incorrect

D. Missing catch block for exceptions

Solution

1. Step 1: Check PDO connection and error mode

   The connection string and error mode setting are correct for throwing exceptions.

2. Step 2: Analyze the SQL statement

   The SQL statement 'INVALID SQL' is not valid SQL, so it triggers a PDOException.

3. Final Answer:

   SQL statement is invalid causing exception -> Option A

4. Quick Check:

   Invalid SQL causes exception = A [OK]

Hint: Invalid SQL triggers PDOException if error mode set [OK]

Common Mistakes:

• Assuming connection string is wrong
• Forgetting error mode is set correctly
• Thinking catch block is missing

5. You want to safely insert user data into a database using PDO and handle errors with exceptions. Which of the following code snippets correctly implements this with prepared statements and exception handling?

hard

A. try {
$pdo = new PDO($dsn, $user, $pass);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (:name, :email)');
$stmt->execute([':name' => $name, ':email' => $email]);
echo 'User added';
} catch (PDOException $e) {
echo 'Error: ' . $e->getMessage();
}

B. $pdo = new PDO($dsn, $user, $pass);
$stmt = $pdo->query('INSERT INTO users (name, email) VALUES ($name, $email)');
echo 'User added';

C. try {
$pdo = new PDO($dsn, $user, $pass);
$stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (?, ?)');
$stmt->execute([$name, $email]);
echo 'User added';
} catch (Exception $e) {
echo 'Error';
}

D. try {
$pdo = new PDO($dsn, $user, $pass);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING);
$pdo->exec('INSERT INTO users (name, email) VALUES ("$name", "$email")');
echo 'User added';
} catch (PDOException $e) {
echo 'Error: ' . $e->getMessage();
}

Solution

1. Step 1: Check for proper error mode setting

   try { $pdo = new PDO($dsn, $user, $pass); $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (:name, :email)'); $stmt->execute([':name' => $name, ':email' => $email]); echo 'User added'; } catch (PDOException $e) { echo 'Error: ' . $e->getMessage(); } sets PDO::ATTR_ERRMODE to ERRMODE_EXCEPTION, enabling exception handling.

2. Step 2: Verify use of prepared statements and parameter binding

   try { $pdo = new PDO($dsn, $user, $pass); $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (:name, :email)'); $stmt->execute([':name' => $name, ':email' => $email]); echo 'User added'; } catch (PDOException $e) { echo 'Error: ' . $e->getMessage(); } uses prepared statements with named parameters and executes safely with user data.

3. Step 3: Confirm exception handling

   try { $pdo = new PDO($dsn, $user, $pass); $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (:name, :email)'); $stmt->execute([':name' => $name, ':email' => $email]); echo 'User added'; } catch (PDOException $e) { echo 'Error: ' . $e->getMessage(); } catches PDOException and outputs the error message, providing safe error handling.

4. Final Answer:

   try { $pdo = new PDO($dsn, $user, $pass); $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); $stmt = $pdo->prepare('INSERT INTO users (name, email) VALUES (:name, :email)'); $stmt->execute([':name' => $name, ':email' => $email]); echo 'User added'; } catch (PDOException $e) { echo 'Error: ' . $e->getMessage(); } -> Option A

5. Quick Check:

   Prepared + ERRMODE_EXCEPTION + try-catch = A [OK]

Hint: Use prepared statements with ERRMODE_EXCEPTION and try-catch [OK]

Common Mistakes:

• Not setting error mode to exceptions
• Using query() with unescaped variables
• Catching generic Exception instead of PDOException
• Using ERRMODE_WARNING which doesn't throw exceptions