# 类常量访问权限控制

现在起支持设置类常量的可见性。可使用private、protected、public权限控制．

```php
class ConstDemo
{
    // 常量默认为 public
    const PUBLIC_CONST_A = 1;

    // 可以自定义常量的可见范围
    public const PUBLIC_CONST_B = 2;
    protected const PROTECTED_CONST = 3;
    private const PRIVATE_CONST = 4;

    // 多个常量同时声明只能有一个属性
    private const FOO = 1, BAR = 2;
}

class ConstDemo1 extends ConstDemo
{
    public function __construct() {
        echo self::PUBLIC_CONST_A;
        echo self::PUBLIC_CONST_B;
        echo self::PROTECTED_CONST;
        echo self::PRIVATE_CONST;
    }
}

 new ConstDemo1();
```

结果：

```php
123
```

由此可见用法和之前的对象属性和对象方法的控制一样．

此外，接口（interface）中的常量只能是 `public`属性：

```php
<?php
interface ICache {
    public const PUBLIC = 0;
    const IMPLICIT_PUBLIC = 1;
}
```


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