Questions about reference counting in php?

Question

When testing php about reference counting, I saw that a variable has a zval container. It contains two attributes, one is refcount and the other is_ref

$array= array( 'meaning' => 'life', 'number' => 42 );
xdebug_debug_zval( 'array' );

When testing the reference count of the array, the expected results are returned

array (refcount=1, is_ref=0),
array (size=2)
'meaning' => (refcount=1, is_ref=0),string ' life' (length=4)
'number' => (refcount=1, is_ref=0),int 42

But when testing objects. Have a question?

class  A{
    private $a1;
    public  $a2;
}

$class1 = new A();

xdebug_debug_zval('class1');

class1:
(refcount=1, is_ref=0),
object(A)[1]
private 'a1' => (refcount=2, is_ref =0),null
public 'a2' => (refcount=2, is_ref=0),null

Don’t quite understand why? Will this be 2? Instead of 1. Instead of like a php array. The expected refcount appears to be 1

And in the php official manual. An explanation of conforming types regarding reference counting.
php reference counting

Things are a little more complicated when it comes to composite types like array and object. Unlike values ​​of scalar type, variables of array and object types store their members or properties in their own symbol tables. .

Answer 1

PHP 5.6.19 显示如下

class X{
    private $a;
    public $b;
}

$x = new X();

xdebug_debug_zval("x");

(refcount=1, is_ref=0),
object(X)[3]
private 'a' => (refcount=1, is_ref=0),null
public 'b' => (refcount=1, is_ref=0),null

PHP Version:7.0.4

相同代码测试结果

x:
(refcount=1, is_ref=0)
object(X)[3]
  private 'a' => (refcount=0, is_ref=0)null
  public 'b' => (refcount=0, is_ref=0)null 

PHP7中简单值直接使用zval容器存储，不再使用指向zval的指针，
也就是说PHP7操作变量是直接操作zval结构体，简单值不再参与垃圾回收
页不再参与引用计数,简单值得赋值直接拷贝，而不再计算引用计数

echo "<hr>PHP7简单值";
$m = 1;
$n = $m;
xdebug_debug_zval("m");
xdebug_debug_zval("n");

PHP7简单值m:
(refcount=0, is_ref=0)int 1
n:
(refcount=0, is_ref=0)int 1

如果简单值被引用了，则结果如下

echo "<hr>PHP7简单值被引用";
$m = 1;
$n = &$m;
xdebug_debug_zval("m");
xdebug_debug_zval("n");

m:
(refcount=2, is_ref=1)int 1
n:
(refcount=2, is_ref=1)int 1

可见当n引用m后，n、m指向同一个变量，引用计数为2，引用为true

unset($n);
m:
(refcount=1, is_ref=1)int 1
n:
(refcount=0, is_ref=0)*uninitialized*

可见unset掉n后，原变量的引用计数减为1，引用仍为true
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