How are php variables implemented?

How to implement PHP variables: first declare a zval pointer and apply for a piece of memory; then set the value to bar through the [ZVAL_STRING] macro; finally add this zval to the current symbol table, and Define its label as foo.

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How to implement php variables:

Introduction

In the official PHP implementation, all variables are saved using the same data structure (zval), and this structure also represents each variable in PHP. data type. It contains not only the value of the variable, but also the type of the variable. This is the core of PHP's weak typing.

• Scalar type: boolean, integer, float(double), string

• Composite type: array, object

• Special types: resource, NULL

Variable storage structure

PHP uses the zval structure in the kernel. To store variables, its definition is in the Zend/zend.h file. It is short and concise, consisting of only four members:

struct _zval_struct {
    zvalue_value value; /* 变量的值 */
    zend_uint refcount__gc;
    zend_uchar type;    /* 变量当前的数据类型 */
    zend_uchar is_ref__gc;
};
typedef struct _zval_struct zval;
//在Zend/zend_types.h里定义的：
typedef unsigned int zend_uint;
typedef unsigned char zend_uchar;

• refcount__gc represents the reference count 1

• is_ref__gc indicates whether it is a reference 0

• value stores the value of the variable

• type the specific type of the variable

Type of variable

The type field of the zval structure is the most critical field to implement weak typing. The value of type can be: IS_NULL ,IS_BOOL,IS_LONG,IS_DOUBLE,IS_STRING,IS_ARRAY,IS_OBJECT,IS_RESOURCE. It's easy to understand literally. They are just unique identifiers of types, and different values ​​are stored in the value field according to different types. In addition, the types defined with them are IS_CONSTANT and IS_CONSTANT_ARRAY.

Example

<?php
$foo = &#39;bar&#39;;
?>

The above is an example of PHP language, create a variable and set its value to 'bar', steps: Create a zval structure, and set its type. Set value to 'bar'. Add it to the symbol table of the current scope so that users can use this variable in PHP. The specific code is:

{
    zval *fooval;
    MAKE_STD_ZVAL(fooval);
    ZVAL_STRING(fooval, "bar", 1);
    ZEND_SET_SYMBOL( EG(active_symbol_table) ,  "foo" , fooval);
}

First, we declare a zval pointer and apply for a piece of memory. Then set the value to bar through the ZVAL_STRING macro. The function of the last line is to add this zval to the current symbol table and define its label as foo, so that users can use it through $foo in the code.

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