Lesson 16 — Robustness

Error and Exception Handling

PHP exceptions let a script signal and gracefully respond to runtime problems, rather than crashing with an unhandled fatal error.

try, catch, throw

Code that might fail goes inside a try block. throw raises an exception object, which a matching catch block can handle without halting the rest of the script.

PHP — Exception handling 
function divide(float $a, float $b): float {
    if ($b === 0.0) {
        throw new InvalidArgumentException("Cannot divide by zero");
    }
    return $a / $b;
}

try {
    echo divide(10, 0);
} catch (InvalidArgumentException $e) {
    echo "Error: " . $e->getMessage();
} finally {
    echo "\nCalculation attempted.";
}

Common Mistakes

  • Catching Exception too broadly, hiding the specific type of error that actually occurred.
  • Forgetting that finally always runs, whether or not an exception was thrown or caught.
  • Throwing plain strings instead of proper Exception objects, which PHP does not support directly.
  • Not creating custom exception classes for domain-specific errors, making error handling less precise across a larger application.

Professional Tip

Create custom exception classes (extending PHP's built-in Exception) for meaningful application errors, like class InsufficientFundsException extends Exception {}. This lets calling code catch specific problems distinctly rather than treating every failure identically.

Your Turn

Create a custom exception class called InsufficientFundsException, and use it inside a withdraw() function that throws it when a withdrawal amount exceeds the available balance.

Mini Quiz

In a try/catch/finally block, when does the finally block execute?