PHP类中动态方法和静态方法调用写法的不解，希望版主大大指导

<?php<br />//error_reporting(0);<br />class A<br />{<br />	public $db='OK';<br />	private static $_instance;<br />	public static function getInstance()<br />    {<br />		if(!(self::$_instance instanceof self)){<br />			self::$_instance = new self;<br />		}<br />		return self::$_instance;<br />	}<br />	//维持find方法中this指针的写法不变<br />	public function find()<br />	{<br />		echo $this->db;<br />	}<br />}<br />class B extends A<br />{<br />	static function ex()<br />	{<br />		self::getInstance()->find();	<br />	}<br />}<br />$b=new B();<br />$b->find();<br />echo "<br>";<br />//第二种方法<br />B::ex();<br />?>