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What is the difference between a PHP error and a PHP exception? How does a try catch block work in PHP, and how do you catch and handle a specific exception that might be thrown? How does the finally block work alongside try and catch, and what are appropriate use cases for it? How do you create and throw a custom exception class in PHP, and why would you define your own exception types instead of using PHP's built in generic Exception class? How do you set a global exception handler and error handler in PHP to consistently catch and log any uncaught exceptions or errors across an entire application? What best practices should guide how an application logs, reports, and responds to exceptions differently across development and production environments?
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What is exception chaining in PHP, and how does the previous exception parameter help preserve the original context when re throwing an error?

Intermediate

Answer

Exception chaining lets you pass an original exception as the previous parameter when constructing and throwing a new exception, preserving the full chain of what actually caused the error even as it gets wrapped in higher level, more descriptive exception types as it propagates up through different layers of your application, which is extremely valuable for debugging since you can trace the exact root cause rather than only seeing the final, more generic exception message.
try {
    $pdo->query($sql);
} catch (PDOException $e) {
    throw new DatabaseException('Failed to fetch user data', previous: $e);
}
Real-world example A developer investigating a reported bug traces a high level DatabaseException all the way back to its original underlying PDOException using the exception chain, immediately identifying the specific SQL error that ultimately caused the problem.

Common follow-ups: How do you access the previous exception from within a catch block?;Does exception chaining affect the stack trace shown when logging an error?

PDO & Databases;Error & Exception Handling
All 7 Error & Exception Handling questions