内容来自于《Extending and Embedding PHP》- Chaper 3 - Memory Management，加上自己的理解，对php中变量的引用计数、写时复制，写时改变，写时复制和改变做个”翻译“。
看下面的内容之前先对zval这个结构体做个了解
<code class="hljs thrift" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 255);">typedef</span> <span class="hljs-class"><span class="hljs-keyword" style="color: rgb(0, 0, 255);">struct</span> _<span class="hljs-title" style="color: rgb(163, 21, 21);">zval_struct</span> </span>{
zvalue_value value;
zend_uint refcount;
zend_uchar type;
zend_uchar is_ref;
} zval;</code>zval结构体中共有4个元素，value是一个联合体，用来真正的存储zval的值，refcount用来计数该zval被多少个变量使用，type表示zval所存储的数据类型，is_ref用来标志该zval是否被引用。
<code class="hljs xml" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="php"><span class="hljs-preprocessor" style="color: rgb(43, 145, 175);"><?php </span>
<span class="hljs-variable">$a</span> = <span class="hljs-string" style="color: rgb(163, 21, 21);">'Hello World'</span>;
<span class="hljs-variable">$b</span> = <span class="hljs-variable">$a</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">unset</span>(<span class="hljs-variable">$a</span>);
<span class="hljs-preprocessor" style="color: rgb(43, 145, 175);">?></span></span></span></code>我们一起来剖析下上面这段代码：
$a = 'Hello World';首先这句代码被执行，内核创建一个变量，并分配12字节的内存去存储字符串'Hello World'和末尾的NULL。$b = $a;接着执行这句代码，执行这句的时候内核里面发生了什么呢？
$a所指向的zval中的refcount进行加1操作。将变量$b指向$a所指向的zval。
在内核中大概是这样的，其中active_symbol_table是当前的变量符号表
<code class="hljs clojure" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"> <span class="hljs-collection">{
zval *helloval;
MAKE_STD_ZVAL<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">helloval</span>)</span><span class="hljs-comment" style="color: green;">;</span>
ZVAL_STRING<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">helloval</span>, <span class="hljs-string" style="color: rgb(163, 21, 21);">"Hello World"</span>, <span class="hljs-number">1</span>)</span><span class="hljs-comment" style="color: green;">;</span>
zend_hash_add<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">EG</span><span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">active_symbol_table</span>)</span>, <span class="hljs-string" style="color: rgb(163, 21, 21);">"a"</span>, sizeof<span class="hljs-list">(<span class="hljs-string" style="color: rgb(163, 21, 21);">"a"</span>)</span>,
&helloval, sizeof<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">zval*</span>)</span>, NULL)</span><span class="hljs-comment" style="color: green;">;</span>
ZVAL_ADDREF<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">helloval</span>)</span><span class="hljs-comment" style="color: green;">;</span>
zend_hash_add<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">EG</span><span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">active_symbol_table</span>)</span>, <span class="hljs-string" style="color: rgb(163, 21, 21);">"b"</span>, sizeof<span class="hljs-list">(<span class="hljs-string" style="color: rgb(163, 21, 21);">"b"</span>)</span>,
&helloval, sizeof<span class="hljs-list">(<span class="hljs-keyword" style="color: rgb(0, 0, 255);">zval*</span>)</span>, NULL)</span><span class="hljs-comment" style="color: green;">;</span>
}</span></code>unset($a);这句代码执行后，内核会将
<code class="hljs xml" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="php"><span class="hljs-preprocessor" style="color: rgb(43, 145, 175);"><?php </span>
<span class="hljs-variable">$a</span> = <span class="hljs-number">1</span>;
<span class="hljs-variable">$b</span> = <span class="hljs-variable">$a</span>;
<span class="hljs-variable">$b</span> += <span class="hljs-number">5</span>;
<span class="hljs-preprocessor" style="color: rgb(43, 145, 175);">?></span></span></span></code>上面这段代码执行完之后，一般肯定希望$a=1,$b=6,但是如果像引用计数那样，$a和$b指向相同的zval，修改$b之后$a不是也变了？
这个具体是怎么实现的呢，我们一起来看下：
$a = 1;内核创建一个zval，并分配4个字节存储数字1。$b = $a;这一步和引用计数中的第二步一样，将$b指向和$a相同的zval，并将zval中的引用计数值refcount加1。$b += 5;关键是这一步，这一步骤发生了什么呢，怎么确保修改之后不影响$a。
get_var_and_separete操作，如果recfount>1,就需要分离就创建新的zval返回，否则直接返回变量所指向的zval，下面看看如何分离产生新的zval。$b所指向zval一样的zval。$b所指向的zval中的refcount计数减1。$b指向新生成的zval。对新生成的zval进行操作，这就是写时复制。
下面看看内核中分离时的主要代码：
<code class="hljs lasso" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"> zval <span class="hljs-subst">*</span>get_var_and_separate(char <span class="hljs-subst">*</span>varname, int varname_len TSRMLS_DC)
{
zval <span class="hljs-subst">**</span>varval, <span class="hljs-subst">*</span>varcopy;
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">if</span> (zend_hash_find(EG(active_symbol_table),
varname, varname_len <span class="hljs-subst">+</span> <span class="hljs-number">1</span>, (<span class="hljs-literal">void</span><span class="hljs-subst">**</span>)<span class="hljs-subst">&</span>varval) <span class="hljs-subst">==</span> FAILURE) {
<span class="hljs-comment" style="color: green;">/* Variable doesn't actually exist fail out */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">return</span> <span class="hljs-built_in" style="color: rgb(0, 0, 255);">NULL</span>;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">if</span> ((<span class="hljs-subst">*</span>varval)<span class="hljs-subst">-></span>is_ref <span class="hljs-subst">||</span> (<span class="hljs-subst">*</span>varval)<span class="hljs-subst">-></span>refcount <span class="hljs-subst"> <span class="hljs-number">2</span>) {
<span class="hljs-comment" style="color: green;">/* varname is the only actual reference,
* or it's a full reference to other variables
* either way: no separating to be done
*/</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">return</span> <span class="hljs-subst">*</span>varval;
}
<span class="hljs-comment" style="color: green;">/* Otherwise, make a copy of the zval* value */</span>
MAKE_STD_ZVAL(varcopy);
varcopy <span class="hljs-subst">=</span> <span class="hljs-subst">*</span>varval;
<span class="hljs-comment" style="color: green;">/* Duplicate any allocated structures within the zval* */</span>
zval_copy_ctor(varcopy);
<span class="hljs-comment" style="color: green;">/* Remove the old version of varname
* This will decrease the refcount of varval in the process
*/</span>
zend_hash_del(EG(active_symbol_table), varname, varname_len <span class="hljs-subst">+</span> <span class="hljs-number">1</span>);
<span class="hljs-comment" style="color: green;">/* Initialize the reference count of the
* newly created value and attach it to
* the varname variable
*/</span>
varcopy<span class="hljs-subst">-></span>refcount <span class="hljs-subst">=</span> <span class="hljs-number">1</span>;
varcopy<span class="hljs-subst">-></span>is_ref <span class="hljs-subst">=</span> <span class="hljs-number">0</span>;
zend_hash_add(EG(active_symbol_table), varname, varname_len <span class="hljs-subst">+</span> <span class="hljs-number">1</span>,
<span class="hljs-subst">&</span>varcopy, sizeof(zval<span class="hljs-subst">*</span>), <span class="hljs-built_in" style="color: rgb(0, 0, 255);">NULL</span>);
<span class="hljs-comment" style="color: green;">/* Return the new zval* */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 255);">return</span> varcopy;
}</span></code>
<code class="hljs xml" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="php"><span class="hljs-preprocessor" style="color: rgb(43, 145, 175);"><?php </span>
<span class="hljs-variable">$a</span> = <span class="hljs-number">1</span>;
<span class="hljs-variable">$b</span> = &<span class="hljs-variable">$a</span>;
<span class="hljs-variable">$b</span> += <span class="hljs-number">5</span>;
<span class="hljs-preprocessor" style="color: rgb(43, 145, 175);">?></span></span></span></code>上面这段代码执行完之后一般希望是：$a == $b == 6。这个又是怎么实现的呢？
$a = 1;这一步骤和写时复制中的第一步一样。$b = &$a;这一步骤内核会将$b指向$a所指向的zval，将zval中的refcount加1，并将zval中的is_ref置为1。$b += 5;这一步骤和写时复制中的第三步骤一样，但是内核中发生的事情却不一样。
$b进行变化的时候，也会执行get_var_and_separate函数，看是否需要分离。(*varval)->is_ref的话也会直接返回$b所指向的zval，不去分离产生新的zval，不管zval的refcount是否>1。$b值，$a的值也就改变了，因为他们指向相同的zval。说道现在聪明的你可能已经看出点问题了，如果一个zval结构体既有refcount计数又有is_ref引用这个时候怎么办？
<code class="hljs xml" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="php"><span class="hljs-preprocessor" style="color: rgb(43, 145, 175);"><?php </span>
<span class="hljs-variable">$a</span> = <span class="hljs-number">1</span>;
<span class="hljs-variable">$b</span> = <span class="hljs-variable">$a</span>;
<span class="hljs-variable">$c</span> = &<span class="hljs-variable">$a</span>;
<span class="hljs-preprocessor" style="color: rgb(43, 145, 175);">?></span></span></span></code>如果出现上面这种情况的时候，如果$a、$b、$c指向同一个zval结构体，进行改变的时候Zend到底去听谁的？其实这个地方不会指向同一个zval了。
如果对一个is_ref = 0 && refcount >1的zval进行写时改变这种赋值形式(就是引用赋值)的时候，Zend会将等号右边的变量分离出来一个新的zval，
对这个zval进行初始化，对之前的zval的refcount进行减1操作，让等号左边的变量指向这个新的zval，refcount进行加1操作，is_ref=1。看看下面这张图片
<code class="hljs xml" style="font-family: 'Courier New', sans-serif !important; line-height: 1.5 !important; font-size: 12px !important; background-color: rgb(245, 245, 245) !important; border: 1px solid rgb(204, 204, 204) !important; padding: 5px !important; border-top-left-radius: 3px !important; border-top-right-radius: 3px !important; border-bottom-right-radius: 3px !important; border-bottom-left-radius: 3px !important; display: block; overflow-x: auto; color: rgb(0, 0, 0); background-position: initial initial; background-repeat: initial initial;"><span class="php"><span class="hljs-preprocessor" style="color: rgb(43, 145, 175);"><?php </span>
<span class="hljs-variable">$a</span> = <span class="hljs-number">1</span>;
<span class="hljs-variable">$b</span> = &<span class="hljs-variable">$a</span>;
<span class="hljs-variable">$c</span> = <span class="hljs-variable">$a</span>;
<span class="hljs-preprocessor" style="color: rgb(43, 145, 175);">?></span></span></span></code>上面这又是另外一种情况，在is_ref = 1的情况下，试图单纯的进行refcount+1操作的时候会分离出来一个新的zval给等号左边的变量，并初始化他，看看下面这张图片
1.《Extending and Embedding PHP》- Chaper 3 - Memory Management.
