PHP Enumeration implementation

Enum.php

<?php

/**
 * Enumeration base class. 
 * 
 * Clients can implement this to create commonly used enum
 * structs with the use of CONSTs.
 *
 * Example:
 * <pre>
 * class Fruit extends Enumeration 
 * {
 *   const APPLE=1;
 *   const ORANGE=2;
 *
 *   static function labels() 
 *   {
 *     return [
 *       self::APPLE=>'Apple',
 *       self::ORANGE=>'Orange',
 *     ];
 *   }
 * }
 * <pre> 
 *
 * To easily use in drop down lists and in key=>value pairs in
 * general where you want value to be a readable and translatable
 * label, implement the {@link labels} method which should return
 * an array of pairs like above.
 *
 * @author John Fragkoulis <john.fragkoulis@gmail.com>
 * @since 0.5
 */
abstract class Enum implements EnumInterface
{
    protected static $_constants;

    /**
     * @return array the array of constants in contant=>value pairs.
     */
    public static function toArray() 
    {
        $className=get_called_class();
        if(self::$_constants===null || !isset(self::$_constants[$className])) {
            $class=new ReflectionClass($className);
            self::$_constants[$className]=$class->getConstants();
        }
        return self::$_constants[$className];
    }

    /**
     * @return array an array of the constants.
     */
    public static function keys() 
    {
        return array_keys(static::toArray());
    }

    /**
     * @return array the array of the constants in contant=>label pairs.
     * @throws CException if used without being implemented by overriding
     * class.
     */
    public static function labels() 
    {
        return array_combine(static::toArray(), static::toArray());
    }

    /**
     * @param mixed $constant
     * @return string the label assigned to a specific constant.
     */
    public static function label($constant) 
    {
        $labels=static::labels();
        return $labels[$constant];
    }
}

EnumInterface.php

<?php

interface EnumInterface
{
    public static function toArray();
    public static function keys();
    public static function labels();
    public static function label($constant);
}

Another approach would be to implement the interface functionality inside a trait called EnumTrait.php and use that trait in your enum class. as such:

class MyEnum implements EnumInterface
{
    use EnumTrait;

    ....
}