<?php
class Enum
{
    private static $values = [];
    private static $valueMap = [];

    private $value;

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

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

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


    /**
     * @return Enum[]
     * @throws \Exception
     */
    public static function getValues()
    {
        $className = get_called_class();
        if (!array_key_exists($className, self::$values)) {
            throw new \Exception(sprintf("Enum is not initialized, enum=%s", $className));
        }
        return self::$values[$className];
    }

    public static function getEnumObject($value)
    {
        if (empty($value)) {
            return null;
        }
        $className = get_called_class();
        return self::$valueMap[$className][$value];
    }

    public static function init()
    {
        $className = get_called_class();
        $class = new \ReflectionClass($className);

        if (array_key_exists($className, self::$values)) {
            throw new \Exception(sprintf("Enum has been already initialized, enum=%s", $className));
        }
        self::$values[$className] = [];
        self::$valueMap[$className] = [];


        /** @var Enum[] $enumFields */
        $enumFields = array_filter($class->getStaticProperties(), function ($property) {
            return $property instanceof Enum;
        });
        if (count($enumFields) == 0) {
            throw new \Exception(sprintf("Enum has not values, enum=%s", $className));
        }

        foreach ($enumFields as $property) {
            if (array_key_exists($property->getValue(), self::$valueMap[$className])) {
                throw new \Exception(sprintf("Duplicate enum value %s from enum %s", $property->getValue(), $className));
            }

            self::$values[$className][] = $property;
            self::$valueMap[$className][$property->getValue()] = $property;
        }
    }
}

/**
 * Пример реализации конкретного класса:
 */
class Format extends Enum
{
    public static $WEB;
    public static $GOST;
}

Format::$WEB = new Format('web');
Format::$GOST = new Format('gost');