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.