php父类调用子类方法

class a{    public function geta()    {        $str = $this->getstr($msg);        return "class a and ".$msg;    }    public function getstr(&$msg){}}class b extends a{    public function getstr(&$msg)    {        $msg = 'class b';        return true;    }}class c extends b{    public function getstr(&$msg)    {       $msg = 'class c';       return true;    }}$object  = new c();echo $object->geta();

输出：class a and class c

多重继承，子类调用父类方法，父类调用子类方法；