static property overiding in php

static.php

<?php

class YiiBase
{
	public static $app;

	public static function test()
	{
		echo self::$app;
	}

	public static function testB()
	{
		echo static::$app;
	}
}

class Yii extends YiiBase
{
	public static $app;
}

Yii::$app = 42;

// self or reference via YiiBase do not give the Yii property
echo YiiBase::$app . "\n";
echo YiiBase::test(). "\n";
echo Yii::test() . "\n";
echo YiiBase::testB() . "\n";

// only direct access or via static works as excpected
echo Yii::$app . "\n";
echo Yii::testB() . "\n";