PHP kernel-detailed introduction to the principles of static variables, constants, and magic constants

This article uses PHP source code, starting from the structure to analyze static variables, constants, and magic constants.

1. Static variables

We all know that static variables are loaded when the PHP script is loaded, that is, 1. They can be called directly without using new objects, 2. And they are static Variables are stored in the public area. Multiple objects of the same class work together to operate a static variable. 3. The memory of static variables will only be released after the script ends. Regarding these three questions, I would like to ask, why?

Let’s start the description below

Let’s first look at its structure for better analysis and understanding.

Static variables are stored in the function structure _zend_execute_data,

In this structure, there are two very critical structures, op_array and symbol_table

1.*symbol_table stores various variables in this class. Every time a new object is created, a new environment space will be opened. For details, see PHP Kernel--A Brief Talk on PHP Soul HashTble Example 2,

##2. The compiled opcode of the function is stored in the *op_array; structure. What is stored is the logic of this function. Every time a new object is created, Sharing a space logic, you will not open up the environment space independently [very critical, achieving static root cause]

#Zend/zend_compiles.h line 384, execution environment structure

struct _zend_execute_data {
	struct _zend_op *opline;
	zend_function_state function_state;
	zend_op_array *op_array;//！！！！！函数编译后的执行逻辑，编译后的opcode二进制代码，称为op_array，公用一个逻辑
	zval *object;
	HashTable *symbol_table;//！！！！！此函数的符号表地址，每次new会开辟一个新的空间《---------
	struct _zend_execute_data *prev_execute_data;
	zval *old_error_reporting;
	zend_bool nested;
	zval **original_return_value;
	zend_class_entry *current_scope;
	zend_class_entry *current_called_scope;
	zval *current_this;  
	struct _zend_op *fast_ret; /* used by FAST_CALL/FAST_RET (finally keyword) */
	call_slot *call_slots;
	call_slot *call;
};

Zend/zend_compiles.h 261 lines, op_array structure code##

struct _zend_op_array {
	/* Common elements */
	zend_uchar type;
...
/* static variables support */
	HashTable *static_variables;//294行 ,静态变量
...
｝

t1() {
$a +=1 ;
static $b +=1;
t1();
t1();
} //加自身共调用3次

The result $a is 1 every time, and $b = 1,2,3

The reasons are as follows:

The symbol table opened by calling the function three times [3 copies

[t_3 execute_data] ---->[symbol_table_3]

[t_2 execute_data] ---->[symbol_table_2]

[t_1 execute_data] ---->[symbol_table_1]

*op_array->*Static variable table [one copy

:

类的变量是存储在 *symbol_table中的，每个都有其作用域，每次实例化都会在开辟一个环境空间（详见Hashtable第二部分的举例）；而静态变量不同，如代码所示，它存储在op_array里边，op_array是什么，编译生成的opcode代码，存储的是函数的逻辑，不管new多少个对象，这个逻辑是公用的，而静态变量也存储在这个结构中，所以实现了同一类的不同对象可以公用一个静态变量，也解释了在PHP层面，静态变量为什么不用new就直接调用。解释了问题一二，

因为静态变量存储在op_array里边，op_array是在脚本执行结束后释放，所以其也在这个时候释放.，解释问题三。

2.常量

首先看下常量与变量的区别，常量是在变量的zval结构的基础上添加了一额外的元素。如下所示为PHP中常量的内部结构。

常量的结构 (Zend/zend_constants.h文件的33行）

typedef struct _zend_constant {
    zval value; /* zval结构，PHP内部变量的存储结构 */
    char *name; /* 常量名称 */
    uint name_len;  
    int flags;  /* 常量的标记如 CONST_PERSISTENT | CONST_CS */
    int module_number;  /* 模块号 */
} zend_constant;