php在类中使用静态方法有几种方式

<?php  
class a {  
    private static function test() {  
        echo &#39;test&#39;;  
    }  
    public function test1() {  
        self::test();  
    }  
}  
$ab = new a();  
$ab->test1();//结果 test

<?php  
class a {  
    private static function test() {  
        echo &#39;test&#39;;  
    }  
    public function test1() {  
        a::test();  
    }  
}  
$ab = new a();  
$ab->test1();//结果 test

<?php  
class a {  
    private static function test() {  
        echo &#39;test&#39;;  
    }  
    public function test1() {  
        static::test();  
    }  
}  
$ab = new a();  
$ab->test1();//结果 test