Error handling with PDO exceptions in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when a PDO exception occurs?

Consider the following PHP code that tries to connect to a database with wrong credentials. What will be the output?

PHP

<?php
try {
  $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'wronguser', 'wrongpass');
  echo "Connected successfully";
} catch (PDOException $e) {
  echo "Connection failed: " . $e->getMessage();
}
?>

A Connected successfully

B Fatal error: Uncaught PDOException: SQLSTATE[HY000] [1045] Access denied for user 'wronguser'@'localhost' (using password: YES)

C Connection failed: SQLSTATE[HY000] [1045] Access denied for user 'wronguser'@'localhost' (using password: YES)

D No output

💡 Hint

Think about what happens when the connection fails and how exceptions are caught.

The PDO constructor throws a PDOException when connection fails. The catch block catches it and prints the message with 'Connection failed: '.

❓ Predict Output

intermediate

What error is raised when accessing a non-existent PDO attribute?

What error will the following PHP code produce?

PHP

<?php
$pdo = new PDO('sqlite::memory:');
echo $pdo->getAttribute(PDO::ATTR_NON_EXISTENT);
?>

A NULL

B Warning: PDO::getAttribute() expects parameter 1 to be integer, string given

C Notice: Undefined class constant 'ATTR_NON_EXISTENT'

D Fatal error: Uncaught Error: Undefined class constant 'ATTR_NON_EXISTENT'

💡 Hint

Check if the constant exists in the PDO class.

The constant PDO::ATTR_NON_EXISTENT does not exist, so PHP throws a fatal error for undefined class constant.

🔧 Debug

advanced

Why does this PDO exception not get caught?

Examine the code below. Why does the exception not get caught by the catch block?

PHP

<?php
try {
  $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'wronguser', 'wrongpass');
  $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  $pdo->query('INVALID SQL');
} catch (PDOException $e) {
  echo "Error caught: " . $e->getMessage();
}
?>

A The exception is not caught because the error mode is set after the connection attempt.

B The exception is not caught because query() does not throw exceptions.

C The exception is caught and the message is printed.

D The code has a syntax error and does not run.

💡 Hint

Think about when the error mode is set and when exceptions are thrown.

The PDO connection fails first but the error mode is set only after connection. The connection error throws an exception before error mode is set, so it is caught. But the invalid query error mode is set after connection, so query() throws a warning, not exception.

❓ Predict Output

advanced

What is the output of this PDO exception handling code?

What will this PHP code output?

PHP

<?php
try {
  $pdo = new PDO('sqlite::memory:');
  $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  $pdo->exec('CREATE TABLE test(id INTEGER PRIMARY KEY)');
  $pdo->exec('INSERT INTO test(id) VALUES(1)');
  $pdo->exec('INSERT INTO test(id) VALUES(1)');
  echo "Insert succeeded";
} catch (PDOException $e) {
  echo "Insert failed: " . $e->getMessage();
}
?>

A Insert failed: SQLSTATE[23000]: Integrity constraint violation: 19 UNIQUE constraint failed: test.id

B No output

C Fatal error: Uncaught PDOException: SQLSTATE[23000]: Integrity constraint violation

D Insert succeeded

💡 Hint

Think about what happens when you insert a duplicate primary key with exceptions enabled.

The second insert violates the UNIQUE constraint on the primary key. Because error mode is set to throw exceptions, the exception is caught and the message is printed.

🧠 Conceptual

expert

Which option correctly demonstrates catching and rethrowing a PDO exception?

You want to catch a PDOException, log the error message, and then rethrow it to be handled elsewhere. Which code snippet does this correctly?

A

<?php
try {
  $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'user', 'pass');
} catch (PDOException $e) {
  throw $e;
  error_log($e->getMessage());
}
?>

B

<?php
try {
  $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'user', 'pass');
} catch (PDOException $e) {
  error_log($e->getMessage());
  throw $e;
}
?>

C

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D

<?php
try {
  $pdo = new PDO('mysql:host=localhost;dbname=testdb', 'user', 'pass');
} catch (PDOException $e) {
  error_log($e->getMessage());
}
throw $e;
?>

💡 Hint

Remember that code after throw is not executed and variables must be in scope.

Option B logs the error message and then rethrows the exception inside the catch block. Option B throws before logging, so logging never happens. Option B misses semicolons causing syntax errors. Option B throws $e outside catch block where $e is undefined, causing error.

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