Error handling with PDO exceptions in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is PDO in PHP?

PDO stands for PHP Data Objects. It is a way to connect to databases in PHP using a consistent interface.

beginner

Why use exceptions for error handling with PDO?

Exceptions let you catch errors cleanly and handle them without stopping the whole program abruptly.

intermediate

How do you enable exceptions for PDO errors?

Set the PDO attribute PDO::ATTR_ERRMODE to PDO::ERRMODE_EXCEPTION when creating the PDO object.

intermediate

What PHP code snippet shows catching a PDO exception?

try { // PDO code here } catch (PDOException $e) { echo 'Error: ' . $e->getMessage(); }

beginner

What happens if you don't catch a PDO exception?

The script stops running and shows a fatal error message, which is not user-friendly.

How do you tell PDO to throw exceptions on errors?

A Call PDO::enableExceptions() method

B Use try-catch without setting anything

C Set PDO::ATTR_DEFAULT_FETCH_MODE

D Set PDO::ATTR_ERRMODE to PDO::ERRMODE_EXCEPTION

You must set the error mode attribute to PDO::ERRMODE_EXCEPTION to enable exceptions.

What class do you catch to handle PDO exceptions?

A Exception

B PDOException

C ErrorException

D RuntimeException

PDO throws exceptions of the class PDOException.

What is the benefit of using try-catch with PDO exceptions?

A It prevents the script from stopping abruptly on errors

B It makes the code run faster

C It automatically fixes database errors

D It disables error messages

Try-catch lets you handle errors gracefully without stopping the script.

What method gets the error message from a PDO exception?

A getError()

B errorInfo()

C getMessage()

D getCode()

getMessage() returns the error message from the exception.

If PDO error mode is not set to exceptions, what is the default behavior?

A PDO returns false on errors without exceptions

B PDO logs errors automatically

C PDO throws exceptions anyway

D PDO crashes the script

By default, PDO returns false on errors and does not throw exceptions.

Explain how to set up PDO to use exceptions for error handling and why it is useful.

Think about setting attributes when creating PDO and catching exceptions.

Describe what happens when a PDO exception is thrown and not caught in your PHP script.

Consider the impact on the program flow and user experience.

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