为什么数组在PHP中为NULL？

Question

<?php    

class ffooo
{
    public $arr;

    function __construct()
    {
        $arr=array();
    }

    function add($val)
    {
        $arr[]=$val;
    }

    function get($ind)
    {
        return $arr[$ind];
    }
}

$cont=new ffooo();
$cont->add("derek",'chmo');
echo $cont->get(0);
var_dump($cont);

Can anybody explain me why my array $arr is NULL after method add($val)? I try to echo array $arr in method "add",and in this method $arr contained come value; but in another method it becomed NULL? What is the magic?I do't understand the logic(

Answer 1

Because it is defined locally only. To use the class member, you must use $this;

$this->arr