oop - PHP：如何在调用$obj->some_attr之前自动调用$obj->some_func()

<?php class test
{
    public $var_a;
    public $var_b;

    public function __get($name)
    {
        //do something ...
        //ext:
        $pre    = substr($name,0,1);
        $var    = substr($name,1);

        if($pre !== '_') return;
        if(!property_exists($this,$var)) return;
        if(!method_exists($this,$name)) return;

        $this->$name();
    }

    protected function _var_a()
    {
        $this->var_a = 'a';
        return $this->var_a;
    }

}

$test   = new test();
$test->_var_a;
$str    = json_encode($test);
var_dump($str);