Introduction to PHP garbage collection mechanism (gc)

PHP compilation principle:

ze (zend engine) calls the lexical analyzer to remove spaces from the PHP code, split it into tokens after comments, and ze calls syntax analysis The processor then processes the token to form opcode, which exists in the form of op array, and finally executes op array to output the result.

When a PHP thread ends, all memory space currently occupied will be destroyed. So if this thread does not end, how to reclaim the memory?

refcount: Reference technology device, which can be understood as the number of pointers pointing to the container.

is_ref: Whether it is referenced (can only be 0 or 1)

Assignment process:

<?php

$a = &#39;aa&#39;;
   
xdebug_debug_zval(a);  //(refcount=1, is_ref=0),string &#39;aa&#39; (length=6)

$b = $a; 

//以下的两个其实是一个变量容器

xdebug_debug_zval(a); //(refcount=2, is_ref=0),string &#39;aa&#39; (length=6)
xdebug_debug_zval(b); //(refcount=2, is_ref=0),string &#39;aa&#39; (length=6)

unset($b);  //对变量容器 refcount 减1xdebug_debug_zval(a); //(refcount=1, is_ref=0),string &#39;aa&#39; (length=6)
xdebug_debug_zval(b); //b: no such symbol  b变量被销毁，指向被断掉，如果对应容器的引用技术为零，那么该块儿内存被回收


$b = $a;

$b = &#39;bb&#39;;

xdebug_debug_zval(a); //(refcount=1, is_ref=0),string &#39;aa&#39; (length=6)
xdebug_debug_zval(b); //(refcount=1, is_ref=0),string &#39;aa&#39; (length=6)  重新申请一个变量容器存储b，a的变量容器引用减1

Reference process:

<?php

$a = &#39;aa&#39;;

xdebug_debug_zval(&#39;a&#39;);  //(refcount=1, is_ref=0),string &#39;aa&#39; (length=2)

$b = & $a;//变量容器的引用技术加1，引用标记置为1xdebug_debug_zval(&#39;a&#39;);  //(refcount=2, is_ref=1),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;);  //(refcount=2, is_ref=1),string &#39;aa&#39; (length=2)


$b = &#39;123&#39;; 

//php会发现，该容器变量是引用（is_ref），所以容器变量不用像赋值那样再申请一个

xdebug_debug_zval(&#39;a&#39;);  //(refcount=2, is_ref=1),string &#39;123&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;);  //(refcount=2, is_ref=1),string &#39;123&#39; (length=2)


unset($b);//变量容器应用计数减1，引用为零

xdebug_debug_zval(&#39;a&#39;);  //(refcount=1, is_ref=0),string &#39;123&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;); // b: no such symbol

If there are more If you unset one reference, will is_ref be set to zero? Then won’t a bug appear? Variable containers are still references. So let's take a look:

<?php


$a = &#39;aa&#39;;

$b = &$a;
$c = &$a;//可以看到引用refCount是3，is_ref永远是1xdebug_debug_zval(&#39;a&#39;); //(refcount=3, is_ref=1),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;); //(refcount=3, is_ref=1),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;c&#39;); //(refcount=3, is_ref=1),string &#39;aa&#39; (length=2)


unset($b);//我们期待的bug没有出现,只是refcount减1，is_ref还是1xdebug_debug_zval(&#39;a&#39;); //(refcount=2, is_ref=1),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;); // b: no such symbol
xdebug_debug_zval(&#39;c&#39;); //(refcount=2, is_ref=1),string &#39;aa&#39; (length=2)

//那php它怎么知道这个容器还有引用，毕竟is_ref仍然是1，不能计数，那么现在refcount就起作用了，是它告诉php，该变量有几个引用，但问题又来了，如果我干点坏事，在引用的时候，又赋值，它会不会有bug

$e = $a;//我们看到期望的bug还是没出现，这时候再赋值，就不像直接赋值那么简单refcount加1了，而是申请了一个新的变量容器

xdebug_debug_zval(&#39;a&#39;); //(refcount=2, is_ref=1),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;e&#39;); //(refcount=1, is_ref=0),string &#39;aa&#39; (length=2)

Can unset and assignment null recycle variables? Many people mistakenly believe that these two can recycle variable space. In fact, they are wrong. null only reduces the space occupied by variables. From a recycling perspective, the container still exists.

<?php

$a = &#39;aa&#39;;

$b = $a;

$b = null;

//又申请了一个变量容器
xdebug_debug_zval(&#39;a&#39;);  //(refcount=1, is_ref=0),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;);  //(refcount=1, is_ref=0),null   变量空间并没被回收

unset($b);

//这时候才释放了b变量容器的空间
xdebug_debug_zval(&#39;a&#39;);  //(refcount=1, is_ref=0),string &#39;aa&#39; (length=2)
xdebug_debug_zval(&#39;b&#39;);  //b: no such symbol

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