关于伪变量的一段示例代码在PHP5.6和7.0下运行结果不同？

Question

在翻看 PHP 手册的时候看到关于伪变量的内容，其中有一份示例代码：

<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        // Note: the next line will issue a warning if E_STRICT is enabled.
        A::foo();
    }
}

$a = new A();
$a->foo();

// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();

// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>

5.6.30 版本下运行结果是

$this is defined (A)
$this is not defined.
$this is defined (B)
$this is not defined.

7.0.9 版本下运行结果是

$this is defined (A)
$this is not defined.
$this is not defined.
$this is not defined.

我是在 PHPStorm 下切换版本运行的，也在一个代码在线运行网站运行代码，也是这个结果。

有了解的同学解答一下吗？谢谢！