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Enumeration methods

Enums (both Pure Enums and Backed Enums) may contain methods, and may implement interfaces. If an Enum implements an interface, then any type check for that interface will also accept all cases of that Enum.

<?php
interface Colorful
{
    public function 
color(): string;
}

enum Suit implements Colorful
{
    case 
Hearts;
    case 
Diamonds;
    case 
Clubs;
    case 
Spades;

    
// Fulfills the interface contract.
    
public function color(): string
    
{
        return 
match($this) {
            
Suit::HeartsSuit::Diamonds => 'Red',
            
Suit::ClubsSuit::Spades => 'Black',
        };
    }

    
// Not part of an interface; that's fine.
    
public function shape(): string
    
{
        return 
"Rectangle";
    }
}

function 
paint(Colorful $c) { ... }

paint(Suit::Clubs);  // Works

print Suit::Diamonds->shape(); // prints "Rectangle"
?>

In this example, all four instances of Suit have two methods, color() and shape(). As far as calling code and type checks are concerned, they behave exactly the same as any other object instance.

On a Backed Enum, the interface declaration goes after the backing type declaration.

<?php
interface Colorful
{
    public function 
color(): string;
}

enum Suitstring implements Colorful
{
    case 
Hearts 'H';
    case 
Diamonds 'D';
    case 
Clubs 'C';
    case 
Spades 'S';

    
// Fulfills the interface contract.
    
public function color(): string
    
{
        return 
match($this) {
            
Suit::HeartsSuit::Diamonds => 'Red',
            
Suit::ClubsSuit::Spades => 'Black',
        };
    }
}
?>

Inside a method, the $this variable is defined and refers to the Case instance.

Methods may be arbitrarily complex, but in practice will usually return a static value or match on $this to provide different results for different cases.

Note that in this case it would be a better data modeling practice to also define a SuitColor Enum Type with values Red and Black and return that instead. However, that would complicate this example.

The above hierarchy is logically similar to the following class structure (although this is not the actual code that runs):

<?php
interface Colorful
{
    public function 
color(): string;
}

final class 
Suit implements UnitEnumColorful
{
    public const 
Hearts = new self('Hearts');
    public const 
Diamonds = new self('Diamonds');
    public const 
Clubs = new self('Clubs');
    public const 
Spades = new self('Spades');

    private function 
__construct(public readonly string $name) {}

    public function 
color(): string
    
{
        return 
match($this) {
            
Suit::HeartsSuit::Diamonds => 'Red',
            
Suit::ClubsSuit::Spades => 'Black',
        };
    }

    public function 
shape(): string
    
{
        return 
"Rectangle";
    }

    public static function 
cases(): array
    {
        
// Illegal method, because manually defining a cases() method on an Enum is disallowed.
        // See also "Value listing" section.
    
}
}
?>

Methods may be public, private, or protected, although in practice private and protected are equivalent as inheritance is not allowed.

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