类常量呢?

<?php

class YourClass
{
    const SOME_CONSTANT = 1;

    public function echoConstant()
    {
        echo self::SOME_CONSTANT;
    }
}

echo YourClass::SOME_CONSTANT;

$c = new YourClass;
$c->echoConstant();

在PHP 8.1中，您可以使用本机枚举。

基本语法如下所示:

enum TransportMode {
  case Bicycle;
  case Car;
  case Ship;
  case Plane;
  case Feet;
}

function travelCost(TransportMode $mode, int $distance): int
{ /* implementation */ } 

$mode = TransportMode::Boat;

$bikeCost = travelCost(TransportMode::Bicycle, 90);
$boatCost = travelCost($mode, 90);

// this one would fail: (Enums are singletons, not scalars)
$failCost = travelCost('Car', 90);

值

默认情况下，枚举不受任何类型的标量支持。因此TransportMode::Bicycle不是0，您不能在枚举之间使用>或<进行比较。

但以下方法是可行的:

$foo = TransportMode::Car;
$bar = TransportMode::Car;
$baz = TransportMode::Bicycle;

$foo === $bar; // true
$bar === $baz; // false

$foo instanceof TransportMode; // true

$foo > $bar || $foo <  $bar; // false either way

支持枚举

你也可以有“支持的”枚举，其中每个枚举案例都由一个int或字符串“支持”。

enum Metal: int {
  case Gold = 1932;
  case Silver = 1049;
  case Lead = 1134;
  case Uranium = 1905;
  case Copper = 894;
}

如果一个案例有一个支持值，所有案例都需要有一个支持值，没有自动生成的值。 注意，受支持值的类型声明在枚举名称之后 备份值为只读 标量值必须是唯一的 值必须是字面量或字面表达式 要读取支持的值，您可以访问value属性:Metal::Gold->value。

最后，被支持的枚举在内部实现了backdenum接口，它公开了两个方法:

从字符串(int |):自我 tryFrom (int |字符串):自我?

它们几乎是等效的，重要的区别是，如果没有找到值，第一个将抛出异常，而第二个将简单地返回null。

// usage example:

$metal_1 = Metal::tryFrom(1932); // $metal_1 === Metal::Gold;
$metal_2 = Metal::tryFrom(1000); // $metal_2 === null;

$metal_3 = Metal::from(9999); // throws Exception

方法

枚举可以有方法，从而实现接口。

interface TravelCapable
{
    public function travelCost(int $distance): int;
    public function requiresFuel(): bool;
}

enum TransportMode: int implements TravelCapable{
  case Bicycle = 10;
  case Car = 1000 ;
  case Ship = 800 ;
  case Plane = 2000;
  case Feet = 5;
  
  public function travelCost(int $distance): int
  {
    return $this->value * $distance;
  }
  
  public function requiresFuel(): bool {
    return match($this) {
        TransportMode::Car, TransportMode::Ship, TransportMode::Plane => true,
      TransportMode::Bicycle, TransportMode::Feet => false
    }
  }
}

$mode = TransportMode::Car;

$carConsumesFuel = $mode->requiresFuel();   // true
$carTravelCost   = $mode->travelCost(800);  // 800000

值清单

Pure Enums和Backed Enums都在内部实现了接口UnitEnum，其中包括(静态)方法UnitEnum::cases()，并允许检索枚举中定义的案例数组:

$modes = TransportMode::cases();

现在$modes是:

[
    TransportMode::Bicycle,
    TransportMode::Car,
    TransportMode::Ship,
    TransportMode::Plane
    TransportMode::Feet
]

静态方法

枚举可以实现自己的静态方法，这些方法通常用于专门的构造函数。

---

这涵盖了基本知识。要了解全部内容，请前往相关RFC，直到该特性在PHP文档中发布。

我也喜欢java的枚举，因此我这样写我的枚举，我认为这是最类似的行为像在java的枚举，当然，如果有人想使用更多的方法从java应该写在这里，或在抽象类，但核心思想是嵌入在下面的代码

class FruitsEnum {

    static $APPLE = null;
    static $ORANGE = null;

    private $value = null;

    public static $map;

    public function __construct($value) {
        $this->value = $value;
    }

    public static function init () {
        self::$APPLE  = new FruitsEnum("Apple");
        self::$ORANGE = new FruitsEnum("Orange");
        //static map to get object by name - example Enum::get("INIT") - returns Enum::$INIT object;
        self::$map = array (
            "Apple" => self::$APPLE,
            "Orange" => self::$ORANGE
        );
    }

    public static function get($element) {
        if($element == null)
            return null;
        return self::$map[$element];
    }

    public function getValue() {
        return $this->value;
    }

    public function equals(FruitsEnum $element) {
        return $element->getValue() == $this->getValue();
    }

    public function __toString () {
        return $this->value;
    }
}
FruitsEnum::init();

var_dump(FruitsEnum::$APPLE->equals(FruitsEnum::$APPLE)); //true
var_dump(FruitsEnum::$APPLE->equals(FruitsEnum::$ORANGE)); //false
var_dump(FruitsEnum::$APPLE instanceof FruitsEnum); //true
var_dump(FruitsEnum::get("Apple")->equals(FruitsEnum::$APPLE)); //true - enum from string
var_dump(FruitsEnum::get("Apple")->equals(FruitsEnum::get("Orange"))); //false

踩在布莱恩·克莱恩的回答上，我想我可能会给出我的5美分

<?php 
/**
 * A class that simulates Enums behaviour
 * <code>
 * class Season extends Enum{
 *    const Spring  = 0;
 *    const Summer = 1;
 *    const Autumn = 2;
 *    const Winter = 3;
 * }
 * 
 * $currentSeason = new Season(Season::Spring);
 * $nextYearSeason = new Season(Season::Spring);
 * $winter = new Season(Season::Winter);
 * $whatever = new Season(-1);               // Throws InvalidArgumentException
 * echo $currentSeason.is(Season::Spring);   // True
 * echo $currentSeason.getName();            // 'Spring'
 * echo $currentSeason.is($nextYearSeason);  // True
 * echo $currentSeason.is(Season::Winter);   // False
 * echo $currentSeason.is(Season::Spring);   // True
 * echo $currentSeason.is($winter);          // False
 * </code>
 * 
 * Class Enum
 * 
 * PHP Version 5.5
 */
abstract class Enum
{
    /**
     * Will contain all the constants of every enum that gets created to 
     * avoid expensive ReflectionClass usage
     * @var array
     */
    private static $_constCacheArray = [];
    /**
     * The value that separates this instance from the rest of the same class
     * @var mixed
     */
    private $_value;
    /**
     * The label of the Enum instance. Will take the string name of the 
     * constant provided, used for logging and human readable messages
     * @var string
     */
    private $_name;
    /**
     * Creates an enum instance, while makes sure that the value given to the 
     * enum is a valid one
     * 
     * @param mixed $value The value of the current
     * 
     * @throws \InvalidArgumentException
     */
    public final function __construct($value)
    {
        $constants = self::_getConstants();
        if (count($constants) !== count(array_unique($constants))) {
            throw new \InvalidArgumentException('Enums cannot contain duplicate constant values');
        }
        if ($name = array_search($value, $constants)) {
            $this->_value = $value;
            $this->_name = $name;
        } else {
            throw new \InvalidArgumentException('Invalid enum value provided');
        }
    }
    /**
     * Returns the constant name of the current enum instance
     * 
     * @return string
     */
    public function getName()
    {
        return $this->_name;
    }
    /**
     * Returns the value of the current enum instance
     * 
     * @return mixed
     */
    public function getValue()
    {
        return $this->_value;
    }
    /**
     * Checks whether this enum instance matches with the provided one.
     * This function should be used to compare Enums at all times instead
     * of an identity comparison 
     * <code>
     * // Assuming EnumObject and EnumObject2 both extend the Enum class
     * // and constants with such values are defined
     * $var  = new EnumObject('test'); 
     * $var2 = new EnumObject('test');
     * $var3 = new EnumObject2('test');
     * $var4 = new EnumObject2('test2');
     * echo $var->is($var2);  // true
     * echo $var->is('test'); // true
     * echo $var->is($var3);  // false
     * echo $var3->is($var4); // false
     * </code>
     * 
     * @param mixed|Enum $enum The value we are comparing this enum object against
     *                         If the value is instance of the Enum class makes
     *                         sure they are instances of the same class as well, 
     *                         otherwise just ensures they have the same value
     * 
     * @return bool
     */
    public final function is($enum)
    {
        // If we are comparing enums, just make
        // sure they have the same toString value
        if (is_subclass_of($enum, __CLASS__)) {
            return get_class($this) === get_class($enum) 
                    && $this->getValue() === $enum->getValue();
        } else {
            // Otherwise assume $enum is the value we are comparing against
            // and do an exact comparison
            return $this->getValue() === $enum;   
        }
    }

    /**
     * Returns the constants that are set for the current Enum instance
     * 
     * @return array
     */
    private static function _getConstants()
    {
        if (self::$_constCacheArray == null) {
            self::$_constCacheArray = [];
        }
        $calledClass = get_called_class();
        if (!array_key_exists($calledClass, self::$_constCacheArray)) {
            $reflect = new \ReflectionClass($calledClass);
            self::$_constCacheArray[$calledClass] = $reflect->getConstants();
        }
        return self::$_constCacheArray[$calledClass];
    }
}

好吧，对于一个简单的java，比如php中的enum，我使用:

class SomeTypeName {
    private static $enum = array(1 => "Read", 2 => "Write");

    public function toOrdinal($name) {
        return array_search($name, self::$enum);
    }

    public function toString($ordinal) {
        return self::$enum[$ordinal];
    }
}

我们称它为:

SomeTypeName::toOrdinal("Read");
SomeTypeName::toString(1);

但我是PHP初学者，语法不好，所以这可能不是最好的方法。我尝试了一些类常量，使用反射从它的值中获得常量名，可能更整洁。

如果你想要类型安全和一堆与该类型匹配的常量，一种方法是为你的枚举创建一个抽象类，然后用一个锁定的构造函数扩展这个类，如下所示:

abstract class DaysOfWeekEnum{
    public function __construct(string $value){
        $this->value = $value; 
    }
    public function __toString(){
        return $this->value;
    }

}
class Monday extends DaysOfWeekEnum{
    public function __construct(){
        parent::__construct("Monday");
    }
}

class Tuesday extends DaysOfWeekEnum{
    public function __construct(){
        parent::__construct("Tuesday");
    }
}

然后你可以让你的方法获取DaysOfWeek的一个实例，并传递给它一个Monday、Tuesday等实例……唯一的缺点是每次你想使用你的枚举时都必须“更新”一个实例，但我发现这是值得的。

function printWeekDay(DaysOfWeek $day){
    echo "Today is $day.";
}

printWeekDay(new Monday());