Trait for Enum's in PHP

Enum.php

<?php

trait Enum
{
    private $value;

    private function __construct($value)
    {
        static $constants;

        if (null === $constants) {
            $rc = new \ReflectionClass(self::class);
            $constants = $rc->getConstants();
        }

        if (!array_key_exists($value, $constants)) {
            $expected = implode('", "', array_keys($constants));
            throw new \RuntimeException(sprintf('Unexpected value "%s", expected "%s"', $value, $expected));
        }

        $this->value = $value;
    }

    static public function __callStatic($name, $arguments)
    {
        return new self($name);
    }

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