Enums

Enums (8.1) define a closed set of possible values. Pure enums have only cases; backed enums map each case to an int or string for storage and serialization, with from() (throws on invalid value) and tryFrom() (returns null). Enums can implement interfaces, have methods and constants, and work perfectly with match. Use them for statuses, roles and other finite sets instead of string constants.

<?php
enum OrderStatus: string
{
    case Pending = 'pending';
    case Paid = 'paid';
    case Shipped = 'shipped';
    case Cancelled = 'cancelled';

    public function label(): string
    {
        return match ($this) {
            self::Pending => 'Awaiting payment',
            self::Paid => 'Paid',
            self::Shipped => 'On its way',
            self::Cancelled => 'Cancelled',
        };
    }

    public function canTransitionTo(self $next): bool
    {
        return in_array($next, match ($this) {
            self::Pending => [self::Paid, self::Cancelled],
            self::Paid => [self::Shipped, self::Cancelled],
            default => [],
        }, true);
    }
}

$status = OrderStatus::from($row['status']);   // from DB
$maybe  = OrderStatus::tryFrom($_GET['status'] ?? '') ?? OrderStatus::Pending;
echo json_encode(['status' => $status]);       // {"status":"paid"}

Common pitfalls

• Using from() on untrusted input without handling ValueError — use tryFrom().
• Comparing enum cases to strings ($status === 'paid') — compare to cases.
• Enums cannot have state (properties); keep data in other objects.

Key points

• Enums model closed sets of values
• Backed enums serialize to int/string
• Enums can have methods and implement interfaces

Further reading: PHP manual: enumerations