PHP Enums: Complete Guide to Best Practices (PHP 8.1+)

PHP added enums in version 8.1 (released 2021). Many other languages - Java, C#, Kotlin - had them for years. This post is the complete guide: what enums are, how they work, when to use them, when not to, and the best practices I have collected working with them across production codebases at SCAYLE, The Quality Group, and HPE/BMW.

What Are Enums?

The basic idea of an enum is to limit a value to a fixed, known set. Instead of passing the integer 1 or the string "active" around your codebase and trusting that callers know what values are valid, you declare the valid values explicitly and let the type system enforce them.

PHP enums are technically objects and bring capabilities normally reserved for classes - they can implement interfaces, use traits (though you should avoid traits), and define methods. A basic enum looks like this:

<?php
enum Status {
    case ACTIVE;
    case INACTIVE;
    case PENDING;
}

Pure Enums vs Backed Enums

The enum above is a pure enum - cases have no underlying value. A backed enum assigns a scalar value (string or int) to each case:

<?php
enum Status: string {
    case ACTIVE = "active";
    case INACTIVE = "inactive";
    case PENDING = "pending";
}

Once you introduce a backed enum, all cases must have a value - mixing pure and backed cases is not allowed. Backed enums are useful when you need to serialize/deserialize values (e.g. from a database column or API payload). However, overusing backed enums can recreate the same problems as the constant-based approach - see best practices below.

Why Enums Over Constants or Primitive Values?

Consider these two approaches to representing card suits:

<?php
// Constants approach
const HEART = 1;
const SPADE = 2;
const CLUB = 3;
const DIAMOND = 4;

// Enum approach
enum Suit {
    case Heart;
    case Spade;
    case Club;
    case Diamond;
}

With constants, the values 1, 2, 3, 4 have no inherent meaning. If you also use integers for statuses (1 = DRAFT, 2 = PUBLISHED...), you introduce ambiguity - nothing stops someone passing a status integer where a suit is expected. Enums eliminate this: the type system rejects the wrong type entirely.

Beyond readability, enums also solve the equality problem. Before 8.1, simulating enums with objects meant two "equal" values were different object instances:

<?php
$a = new MyEnum(EnumInterface::ACTIVE);
$b = new MyEnum(EnumInterface::ACTIVE);
var_dump($a === $b); // false

With enums, enum cases are singletons - the same case is always the same instance:

<?php
$c = Status::ACTIVE;
$d = Status::ACTIVE;
var_dump($c === $d); // true

You can also use instanceof to check whether a value is a specific enum type, which was impossible with the constant-based approach.

How Were Enums Simulated Before PHP 8.1?

The most common pattern was interface constants:

<?php
interface StatusInterface {
    public const ACTIVE = 'active';
    public const INACTIVE = 'inactive';
    public const PENDING = 'pending';
}

class MyEnum implements StatusInterface {
    public function __construct(private string $value) {}
}

$a = new MyEnum('some invalid value'); // technically valid, semantically wrong

The problem: nothing prevents passing an invalid string. You either add validation boilerplate everywhere, or accept unexpected behaviour. Real enums solve this at the type level with zero extra code.

When Not to Use Enums

Enums are the right tool when you have a small, stable set of values. Ask yourself:

• Does the set of valid values change frequently?
• Could the set grow unboundedly (e.g. user-defined categories)?
• Is performance/memory critical (e.g. real-time systems processing millions of values per second)?
• Do you need to store values you don't know at compile time?

If yes to any of the above, enums are likely the wrong tool. A database table, a value object, or a simple string type may serve better.

Best Practices from Production Experience

1. Prefer pure enums over backed enums by default. Backed enums are useful for serialisation (database persistence, API responses), but overusing them recreates the constant-based approach. If you find yourself reaching for backed enums everywhere, pause and check whether you actually need the scalar value - or whether you are just carrying an old habit.

2. Never compare enums against scalar values. If you have a backed enum and a raw string comes in from user input or a database, convert it to an enum first (Status::from('active')), then compare enum-to-enum. Comparing Status::ACTIVE->value === $someString defeats the purpose of type safety.

3. Keep enums thin. Enums can have methods, but they should describe values - not contain business logic. A simple label() or from()-style translation method is fine. Complex behaviour belongs in a service that accepts the enum as a parameter.

4. Don't let backed values bleed into business logic. Backed values ("active", 1) should exist only at the persistence/serialisation boundary. Inside the application, work with the enum case. If your business logic contains === 'active', you have a leak.

5. Migrate simulated enums carefully. Most constant-based "enums" can be replaced directly, but some need attention - especially if the values are stored in a database or sent over an API. Map the migration in steps: introduce the enum type, add conversion at the boundary, then remove the constants.

6. Use from() vs tryFrom() deliberately. Status::from('invalid') throws a ValueError. Status::tryFrom('invalid') returns null. Use from() at trusted boundaries (your own DB), tryFrom() at untrusted boundaries (user input, external APIs) and handle the null explicitly.

Enums and Database Columns

Backed enums map cleanly to database enum or string columns. Rather than converting back and forth with custom logic, you define the mapping once at the enum level:

<?php
enum Status: string {
    case ACTIVE = "active";
    case INACTIVE = "inactive";
    case PENDING = "pending";
}

// Reading from DB:
$status = Status::from($row['status']);

// Writing to DB:
$query->set('status', $status->value);

This keeps the conversion logic in one place and gives you full type safety inside the application layer.

Conclusion

Enums are one of the best additions to PHP in recent years. They replace ambiguous constant patterns with type-safe, comparable, serialisable values that the language itself enforces. The key discipline is keeping them thin: enums describe values, services use them. Avoid leaking backed values into business logic, prefer pure enums when you don't need serialisation, and migrate existing constant-based patterns carefully.

If you are upgrading a legacy codebase and want advice on migrating constant-based enums safely, get in touch.