为什么魔术方法'__get'被调用了两次？

Question

I have seen some code like below, and it is strange that the __get method has been called twice, why?

class Foo {
    private $bar;

    function __get($name){
        echo "__get is called!";
        return $this->$name;
    }

    function __unset($name){
        unset($this->$name);
    }
}
$foo = new Foo;
unset($foo->bar);
echo $foo->bar;

Attention: unset($foo->bar) will not call the __get.

Answer 1

For me, it looks like a bug. Put some debugging code (the following) and see the result:

<?php

class Foo {
    private $bar;

    function __get($name){
        echo "__get(".$name.") is called!
";
        debug_print_backtrace();
        $x = $this->$name;
        return $x;
    }

    function __unset($name){
        unset($this->$name);

        echo "Value of ". $name ." After unsetting is 
";
        echo $this->$name;
        echo  "
";
    }
}
echo "Before
";
$foo = new Foo;
echo "After1
";
unset($foo->bar);
echo "After2
";
echo $foo->bar;
echo "After3
";
echo $foo->not_found;
?>

The result is:

Before
After1
Value of bar After unsetting is
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:17]
#1  Foo->__unset(bar) called at [E:\temp\t1.php:24]
PHP Notice:  Undefined property: Foo::$bar in E:\temp\t1.php on line 9

After2
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:26]
__get(bar) is called!
#0  Foo->__get(bar) called at [E:\temp\t1.php:9]
#1  Foo->__get(bar) called at [E:\temp\t1.php:26]
PHP Notice:  Undefined property: Foo::$bar in E:\temp\t1.php on line 9
After3
__get(not_found) is called!
#0  Foo->__get(not_found) called at [E:\temp\t1.php:28]
PHP Notice:  Undefined property: Foo::$not_found in E:\temp\t1.php on line 9