meet it

Name each problem with its own exception class.

Throwing a plain Exception for everything is like a shop alarm with one sound for fire, flood and burglars. A is a class that extends Exception, and its name says what went wrong: class OutOfStock extends Exception {}. The empty body is enough.

Now catch can choose problems by type. Write several catch blocks in a row. PHP tries them top to bottom and runs the first whose type fits, so put the most specific first.

Alongside the InvalidArgumentException you already know, PHP ships , for problems that only appear while the program runs.

<?php
class OutOfStock extends RuntimeException {}
function buy(string $item, int $stock): string {
    if ($stock === 0) {
        throw new OutOfStock("Sorry, $item is sold out.");
    }
    return "One $item, coming up.";
}
foreach (["scone" => 3, "eclair" => 0] as $item => $stock) {
    try {
        echo buy($item, $stock) . "\n";
    } catch (OutOfStock $e) {
        echo $e->getMessage() . "\n";
    }
}

get_class($e) gives the name of an exception's class, which helps when you debug.

guess first

Two catch blocks could fit. Which one runs?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

<?php
class OutOfStock extends RuntimeException {}
try {
    throw new OutOfStock("No more crumpets");
} catch (RuntimeException $e) {
    echo "Something went wrong: " . get_class($e) . "\n";
} catch (OutOfStock $e) {
    echo "Sold out: " . $e->getMessage() . "\n";
}

your turn

Your turn. Refuse impossible ages.

Name your exception, like BadAge, and type that name in both of the first two blanks. Then try any age, sensible or silly, in the last one.

Your code

press run to see what happens

fix it

The order form crashes instead of explaining. Fix it.

Broken code to fix

press run to see what happens

The run stops with an uncaught InvalidArgumentException. There is a catch, so why does it not step in?
A catch only catches its own type and that type's children. An InvalidArgumentException is not an OutOfStock.
On line 11, change OutOfStock to InvalidArgumentException, then run it.

nailed it

you gave every problem its own name. :D

A custom exception is a class that extends Exception.
catch blocks are tried in order, so put specific types first.
PHP ships InvalidArgumentException and RuntimeException, ready to extend.

new words in your collection

custom exception
RuntimeException

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