PHP variable principle

1.php as a weakly typed language does not need to explicitly indicate the type of variables, but php variables also have types. php variables include the following 8 types of variables (three major categories)

　a. Scalar type: boolean, integer, float (double), string

b. Composite type: array, object

c. Special type: resource, null

2.php uses c language to implement the principle of variables

a. Variable storage structure

typedef struct _zval_struct zval;
...
struct _zval_struct {
    /* Variable information */
    zvalue_value value;     /*存储变量的值，是一个union类型*/
    zend_uint refcount__gc;/*变量的引用计数，默认为1*/
    zend_uchar type;    /*变量的类型，为IS_NULL、IS_BOOL、IS_LONG、IS_DOUBLE、IS_STRING、IS_ARRAY、IS_OBJECT和IS_RESOURCE之一*/
    zend_uchar is_ref__gc;/*表示是否为引用*/
};

　b. The value of the stored variable zvalue_value is 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;

The purpose of using union instead of struct here is to save memory space, thinking that a variable can only represent one type at the same time

Reference: tipi open source project http://www .php-internals.com/book/?p=chapt03/03-01-00-variables-structure

The above introduces the principle of PHP variables, including aspects of content. I hope it will be helpful to friends who are interested in PHP tutorials.