Extension and embedding of php--php internal variables_PHP tutorial

I previously introduced the internal life cycle of PHP and the thread safety mechanism of the Zend engine. This article here mainly introduces how the internal variables of PHP are implemented.

After understanding these implementation methods, it feels very helpful for writing PHP, especially for PHP extension development.

PHP is a relatively loosely typed language. Compared with C, you don’t need to give the type before using the variable, you can just use it directly. In order to achieve this, PHP has to do some work on the definition of data types.

Data type:

The most basic type is called zval or Zend Value, defined in the Zend/zend.h header file. typedef struct _zval_struct {
zvalue_value value;
zend_uint refcount;
zend_uchar type;
zend_uchar is_ref;
} zval; Among them, zvalue_value is defined as follows: typedef union _zvalue_value {
long lval;
double dval;
struct {
char *val;
int len;
} str;
HashTable *ht;
zend_object_value obj;
} zvalue_value; This is a union. There are two possibilities when using union. One is when all variables share a memory space, and the other is when one of the types needs to be selected from n.
Zend defines 8 basic data types. These eight are basically seen in other languages, so only the more special types are explained: IS_NULL: non-valueIS_BOOL: IS_LONGIS_DOUBLEIS_STRING: The allocated space needs to be length + 1IS_ARRAY: The array of php is actually a hashtable, including label and dataIS_OBJECT: On the basis of the array, methods, access modifiers, domain constants and special are added Event handler IS_RESOURCE: For example, file handles or mysql handles are stored in the type of zval and have separate correspondences with zvalue_value Note that there is a comparison of the two type judgment functions below: and
The first piece of code is written in C, and the second piece of code is written in PHP style. Note that the Zend header file provides many macros for zval processing. It is best to use them. Here, Z_TYPE_P(foo) is used. There are also Z_TYPE() and Z_TYPE_PP() corresponding to zval and zval** respectively php_printf() is based on printf and has made some optimizations for SAPI and PHP output mechanisms.

Data value

The values ​​of different types of zval can be obtained through some macros: BVAL(): BooleanLVAL(): longDVAL(): double This function uses Z_TYPE to perform type judgment for three different zval types. Then use the corresponding value extraction macro to obtain the value.
The processing of string is slightly more special: two macros Z_STRVA and Z_STRLEN are needed to read the value and length respectively. This can also be seen from the definition of the string type, which is Composed of characters and length.
Access to the array uses the ARRVAL series: Z_ARRVAL(zv), Z_ARRVAL_P(pzv), Z_ARRVAL_PP(ppzv). In some versions of PHP source code, HASH_OF() is equivalent to Z_ARRVAL_P, but this macro has gradually become less used.
For Object: OBJ_HANDLE returns the object handle identifier OBJ_HT handle table OBJCE class definition OBJPROP attribute hash table OBJ_HANDLER operates specific processing methods in OBJ_HT For resources, just use the macro RESVAL directly.

Data creation:

想要创造一个变量并分配空间的malloc(sizeof(zval))在php这里并不可行。应该使用MAKE_STD_ZVAL(pzv), 它对空间的分配进行了优化，并且会自动的初始化refCount(表示这个变量被引用的次数)和is_ref(是否是强制引用)这两个性质。注意它的输入是一个指针.
ALLOC_INIT_ZVAL()也可以进行初始化,不同之处在于把zval*的值设为了NULL.
在设置不同类型的值的时候有很多形式,左边是比较简略的形式,右侧则是展开的形式： ZVAL_NULL(pvz); Z_TYPE_P(pzv) = IS_NULL;
ZVAL_BOOL(pzv, b); Z_TYPE_P(pzv) = IS_BOOL;
Z_BVAL_P(pzv) = b ? 1 : 0;
ZVAL_TRUE(pzv); ZVAL_BOOL(pzv, 1);
ZVAL_FALSE(pzv); ZVAL_BOOL(pzv, 0);
ZVAL_LONG(pzv, l); Z_TYPE_P(pzv) = IS_LONG;
Z_LVAL_P(pzv) = l;
ZVAL_DOUBLE(pzv, d); Z_TYPE_P(pzv) = IS_DOUBLE;
Z_DVAL_P(pzv) = d;
对于字符串的处理要特殊一些,提供了一个单独的参数dup. 这个参数决定了是否创建一个字符串的副本.举个例子 zval * pzva; ZVAL_STRING(pzval,"hello world",1); 由于“hello_world”是一个常量字符串，直接对它进行操作显然不合适，所以把dup设为1的话，会自动的给它创建一个副本，然后再赋给pzval. 这使得整个过程更加简洁。

ZVAL_STRINGL(pzv,str,len,dup); Z_TYPE_P(pzv) = IS_STRING;
Z_STRLEN_P(pzv) = len;
if (dup) {
Z_STRVAL_P(pzv) =
estrndup(str, len + 1);
} else {
Z_STRVAL_P(pzv) = str;
}
ZVAL_STRING(pzv, str, dup); ZVAL _STRINGL(pzv, str,
strlen(str), dup); 注意dup如果设为1的话就是申请新的空间并且拷贝内容，而不是一个shaddow copy。 ZVAL_RESOURCE(pzv, res); Z_TYPE_P(pzv) = IS_RESOURCE;
Z_RESVAL_P(pzv) = res;

数据的存储

数据的存储都在符号表中。 symbol table，每当创建一个新的变量的时候，Zend都保存这个值到这个内部的数组中去。 符号表在RINIT之前创建，在RSHUTDOWN之后销毁。
当用户空间的函数或对象方法被调用的时候，会创建一个新的符号表，生命与函数执行时间相同。 在Zend/zend_gblobals.h中定义了两个元素：
通过EG(symbol_table) 的方式可以访问符号表。感觉跟$GLOBALS似的。 注意到EG(symbol_table)这个宏返回的不是指针，必须加上&。 下面的这个对比非常的有趣： In PHP:
针对这段php的代码，C中一共做了如下这些事：
In C:
所谓active_symbol_table指的是程序执行当前的符号表，在进入一个函数之后，会有它自己对应的符号表，就类似于C中针对一个函数自己的栈空间。而当退出了函数之后，它的符号表会被销毁，这时候又回到了下面这个状态： EG(active_symbol_table) == &EG(symbol_table)， 这个时候并没有进入函数。

数据的获取： 在获取数据的时候，比较多的是使用zend_hash_find()函数：

这个函数首先查找符号表，找到名字为“foo”的变量，然后返回到fooval中。下面着重解释两个问题：

为什么要声明一个zval ** fooval 然后还要通过&fooval并且转换为(void **)的形式？为什么要用sizeof("foo") 对第一个问题，要考虑到我们寻找的目标是一个zval*,所以要把它看作一个整体。利用这种写法可以避免编译告警。 第二个问题，使用sizeof(label)主要是为了表示字符串常量label的尾部，这里使用4也是可以的，但是通用性不够。

数据转换： 仅仅是说一下有这个功能，比如convert_to_string(zval *value)可以把zval转换为字符串。

以上就是php内部变量的一些介绍，为了能够区分不同的类型、设置获取变量值以及在符号表中增加和查找变量，这些知识必不可少。