Type safe enum implementation in PHP

Enum.php

<?php

abstract class Enum {
	
	protected $value;
	
	public function getValue()
	{
		return $this->value;
	}
	
	private function __construct($name, $childClass)
	{
		$constant = "$childClass::$name";
		if (defined($constant)) {
			$this->value = constant($constant);
		} else {
			throw new \InvalidStateException($name . ' is not valid option for ' . $childClass);
		}
	}

	public function __toString() {
		return $this->value;
	}
	
	public static function __callStatic($name, $params)
	{
		$childClass = get_called_class();
		return new $childClass($name, $childClass);
	}
	
}

// Example usage:

class Family extends Enum {
	const 
		FATHER = 'father',
		MOTHER = 'mother',
		BROTHER = 'brother',
		SISTER = 'sister';
}

$member = Family::FATHER();