PHP 5.3 and typesafe enumerations

Within RFC #0132, typesafe enumerations were added to the XP framework. Because PHP does not support them language-wise, we need a bit of workaround syntax to make them work.

With PHP 5.3 and its so-called "late static binding features", enum declarations can now be written much more compact for the simplest use-case. Example:

  class TransactionType extends Enum {
    public static $NOT_SUPPORTED, $REQUIRED, $SUPPORTS, $NEVER;
  }

Previously, we needed to write this as follows:

  class TransactionType extends Enum {
    public static $NOT_SUPPORTED, $REQUIRED, $SUPPORTS, $NEVER;
        
    static function __static() {
      self::$NOT_SUPPORTED= new self(0, 'NOT_SUPPORTED');
      self::$REQUIRED= new self(1, 'REQUIRED');
      self::$SUPPORTS= new self(2, 'SUPPORTS');
      self::$NEVER= new self(3, 'NEVER');
    }
    
    public static function values() {
      return parent::membersOf(__CLASS__);
    }
  }

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