meet it

An exception can carry extra details, and even another exception.

A message is only text, but a form wants to know which field failed. Give your exception a constructor with an extra readonly property, then hand the message on with parent::__construct($message) so that getMessage() still works.

When a low-level error bubbles up, like a database complaint, it: catch it and throw your own clearer exception, passing the original as the third argument, as in new StorageFailed("...", 0, $e). The 0 is a number code. Later, getPrevious() hands the original back. Linked like this, they form an .

<?php
class InvalidField extends InvalidArgumentException {
    public function __construct(public readonly string $field, string $message) {
        parent::__construct($message);
    }
}
function checkEmail(string $email): void {
    if (filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
        throw new InvalidField("email", "That email does not look right.");
    }
}
try {
    checkEmail("mia-at-example");
} catch (InvalidField $e) {
    echo "Field '$e->field': " . $e->getMessage() . "\n";
}

A form can now put each message beside the right box, because it knows $e->field.

guess first

A missing table, wrapped up. What will this show?

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

<?php
class StorageFailed extends RuntimeException {}
function emptyBasket(PDO $db): void {
    try {
        $db->exec("DELETE FROM basket");
    } catch (PDOException $e) {
        throw new StorageFailed("Could not empty the basket.", 0, $e);
    }
}
try {
    emptyBasket(new PDO("sqlite::memory:"));
} catch (StorageFailed $e) {
    echo $e->getMessage() . "\n";
    echo get_class($e->getPrevious()) . "\n";
}

your turn

Your turn. Tell the shopper how far over budget they are.

Type a price and a budget, both in pence. Try a price above the budget to see the exception carry its extra detail.

Your code

press run to see what happens

fix it

The form says there is a problem, but not what. Fix it.

Broken code to fix

press run to see what happens

No crash, but the message is empty. Line 5 pretends a form was sent with a blank name. Where should the words come from?
The constructor stores $field but never passes a message on to the parent, so getMessage() has nothing to give.
Inside the constructor's curly brackets, add parent::__construct("Please fill in $field.");, then run it.

nailed it

your exceptions carry the full story. :D

Extra readonly properties let an exception carry useful details.
Call parent::__construct($message) so getMessage() still works.
Wrap low-level errors, passing the original as the third argument.

new words in your collection

wrap
exception chain

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