What's the Difference Between Pure Enums and Backed Enums in PHP?

Introduced in PHP 8.1, pure enums and backed enums have the following differences:

1. They're Composed of Different Enum Cases;
2. Backed Enum Cases Have an Additional value Property.

Differences in Composition

Pure Enums:

Pure enums can only contain "Pure Cases" — i.e. a case that has no assigned value:

// PHP 8.1+
enum MyEnum
{
    case Foo;
    case Bar;
    case Baz;
}

Please note that these cases are not implicitly backed by a default value (such as 0 for instance).

Backed Enums:

Backed enums can only contain "Backed Cases" — i.e. a case that's assigned a scalar value:

// PHP 8.1+
enum MyEnum: string
{
    case Foo = 'foo';
    case Bar = 'bar';
    case Baz = 'baz';
}

Please note that these cases do not have auto-generated scalar equivalents (such as a sequence of numbers). Therefore, all cases must have a unique scalar value defined explicitly (which can only be of a single type at a time).

Properties

Both, pure enums and backed enums, have a read-only name property which returns the case-sensitive name of the case itself:

// PHP 8.1+
// ...

echo MyEnum::Foo->name; // 'Foo'

Backed enums, however, have an additional read-only value property that returns the value of the respective case:

// PHP 8.1+
// ...

echo MyEnum::Foo->value; // 'foo'