protected abstract static function ... E_STRICT

Hi, why PHP in Strict mode fires as warning for the declaration of a abstract function: What I want to do is: abstract class Scalar { protected $value; public function __construct($value) { if (static::valid($value)) { $this->value = $value; } else { throw new RuntimeException(); } } /** each child has to have it's own function */ protected abstract static function valid($value); } final class Integer extends Scalar { protected static function valid($value) { return is_int($value); } } final class Float extends Scalar { protected static function valid($value) { return is_float($value) || is_int($value); } } The abstract superclass has no need implement the valid function but the childclasses have to. What I'm having as workaround at the moment: abstract class Scalar { ... protected static function valid($value) { throw new BadMethodCallException('Method should be only called in subclass'); } } Any better solutions for this? Thanks, -- DerOetzi