We pioneered a pattern for exception handling for Zend Framework back as we initially began development on version 2 around seven years ago. The pattern looks like this:

• We would create a marker ExceptionInterface for each package.
• We would extend SPL exceptions and implement the package marker interface when doing so.

What this gave users was the ability to catch in three ways:

• They could catch the most specific exception type by class name.
• They could catch all package-level exceptions using the marker interface.
• The could catch general exceptions using the associated SPL type.

So, as an example:

try {
    $do->something();
} catch (MostSpecificException $e) {
} catch (PackageLevelExceptionInterface $e) {
} catch (\RuntimeException $e) {
}

This kind of granularity is really nice to work with. So nice that some standards produced by PHP-FIG now ship them, such as PSR-11, which ships a ContainerExceptionInterface and a NotFoundExceptionInterface.

One thing we've started doing recently as we make packages support only PHP 7 versions is to have the marker ExceptionInterface extend the Throwable interface; this ensures that implementations must be able to be thrown!

So, what happens when you're writing a one-off implementation of something that is expected to throw an exception matching one of these interfaces?

Why, use an anonymous class, of course!

As an example, I was writing up some documentation that illustrated a custom ContainerInterface implementation today, and realized I needed to throw an exception at one point, specifically a Psr\Container\NotFoundExceptionInterface. I wrote up the following snippet:

$message = sprintf(/* ... */);
throw new class($message) extends RuntimeException implements
    NotFoundExceptionInterface {
};

Done!

This works because RuntimeException takes a message as the first constructor argument; by extending that class, I gain that behavior. Since NotFoundExceptionInterface is a marker interface, I did not need to add any additional behavior, so this inline example works out-of-the-box.

What else are you using anonymous classes for?