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?

You create a custom exception by defining a class that extends PHP's built in Exception class, or a more specific built in exception, optionally adding your own additional properties or methods, and defining custom exception types lets calling code catch and respond to very specific error conditions distinctly, such as differentiating between an InsufficientFundsException and a generic database error, rather than needing to inspect a generic exception's message text to determine what actually went wrong.

class InsufficientFundsException extends Exception {
    public function __construct(private float $shortfall) {
        parent::__construct("Insufficient funds, short by $shortfall");
    }
    public function getShortfall(): float { return $this->shortfall; }
}