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In-depth understanding of the PHP kernel (5) variables and data types - the structure and type of variables, in-depth understanding of the kernel_PHP tutorial
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- 2016-07-12 08:56:27816browse
In-depth understanding of PHP kernel (5) variables and data types - the structure and type of variables, in-depth understanding of the kernel
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The types of programming languages can be divided into two types: strongly typed and weakly typed. PHP is a weakly typed language, but C language is a strongly typed language. Within the official PHP implementation, all variables are stored using the same data structure (zval). This structure represents various data types in PHP. It not only contains the value of the variable, but also the type of the variable. This is the core of PHP's weak typing.
How does the zval structure implement weak type?
1. PHP variable types and storage structures
PHP does not need to specify its data type when declaring and using variables. PHP is a weakly typed language, but PHP has types. PHP has eight data types, which can be divided into three categories: scalar Type: boolean, integer, float (double) string; composite type: array, object; special type: resource, NULL
How does C language implement weak types in PHP?
1. Variable storage structure
The value of the variable is stored in the zval structure as shown below. The zval structure is defined in the Zend/zend.h file and its structure is as follows:
typedef struct _zval_struct zval;
...
struct _zval_struct {
/* Variable information */
zvalue_value value; /* value */
zend_uint refcount__gc;
zend_uchar type; /* active type */
zend_uchar is_ref__gc;
};
There are four fields in the zval structure, their meanings are:
| 属性名 | 含义 | 默认值 |
| refcount_gc | 表示引用计数 | 1 |
| is_ref_gc | 表示是否为引用 | 0 |
| value | 存储变量的值 | |
| type | 变量具体的类型 |
After PHP5.3, a new garbage collection mechanism was introduced. The reference count and referenced field names were changed to refcout_gc and is_ref_gc. Before that, they were refcount and is_ref.
The value of the variable is stored in another structure zvalue_value. See the introduction below for value storage.
2. Variable type
The type field of the zval structure is the most critical field to implement weak typing. The value of type can be one of: IS_NULL, IS_BOOL, IS_LONG, IS_DOUBLE, IS_STRING, IS_OBJECT and IS_RESOURCE. In addition, the types defined with them include IS_CONSTANT and IS_CONSTANT_ARRAY.
2. Storage of variable values
The value of the variable mentioned earlier is stored in the zvalue_value union, and the structure is defined as follows:
typedef union _zvalue_value {
long lval; /* long value */
double dval; /* double value */
struct {
char *val;
int len;
} str;
HashTable *ht; /* hash table value */
zend_object_value obj;
} zvalue_value;
Various types of data will use different methods to store variable values. The corresponding assignment methods are as follows:
General type:
| Variable type | Macros | ||||||||||||||||||
| boolean | ZVAL_BOOL |
|
|||||||||||||||||
| integer | ZVAL_LONG | ||||||||||||||||||
| float | ZVAL_DOUBLE | Z_TYPE_P(z)=IS_BOOL/LONG;Z_LVAL_P(z)=(b)!=0; | |||||||||||||||||
| null | ZVAL_NULL | NULL value variable values do not need to be stored, just mark (zval).type as IS_NUL Z_TYPE_P(z)=IS_NULL; | |||||||||||||||||
| resource | ZVAL_RESOURCE | The storage of resource types is no different from other general variables, but its initialization and storage implementation are different Same as Z_TYPE_P(z) = IS_RESOURCE; Z_LVAL_P(z) = 1; |
字符串
字符串的类型标示和其他数据类型一样，不过在存储字符串时多了一个字符串长度的字段。
struct {
char *val;
int len;
} str;
（存储字符串长度是因为字符串的操作十分频繁，有利于节省时间，是空间换时间的做法）
数组Array
数组是PHP中最常用也是最强大变量类型。数组的值存储在zvalue_value.ht字段中，它是一个HashTable类型的数据。PHP数组使用哈希表来存储关联数据。PHP的哈希表实现中使用了两个数据结构HashTable和Bucket。PHP所有的工作都是由哈希表实现。
对象Object
PHP的对象是一种复合型的数据，使用一种zend_object_value的结构体来存放，其定义如下
typedef struct _zend_object_value {
zend_object_handle handle; // unsigned int类型，EG(objects_store).object_buckets的索引
zend_object_handlers *handlers;
} zend_object_value;
PHP的对象只有在运行时才会被创建，前面介绍了EG宏，这是一个全局结构体由于保存在运行时的数据。其中就包括了用来保存所有被创建的对象的对象池，EG(objects_store)，而object对象值内容的zend_object_handle域就是当前对象在对象池中所在的索引，handlers字段则是将对象进行操作时的处理函数保存起来。
PHP的弱变量容器的实现方式是兼容并包的形式体现。