How to implement weakly typed variables in PHP

PHP is a weakly typed, dynamic language script. When declaring a variable, you do not need to specify the data type it holds. For example:

<?php  
$var = 1;  
$var = "variable";  
$var = 1.00;  
$var = array();  
$var = new Object();

Dynamic variables can be changed during runtime, and there is no need to declare the variable type before use.

Question 1. How does the Zend engine implement this weak type in C?

In fact, variables declared in PHP are stored in ZE using the structure zval.

First we open Zend/zend.h to see the definition of zval:

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;  
};  
   
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;

Zend/zend_types.h:

typedef unsigned char zend_bool;  
typedef unsigned char zend_uchar;  
typedef unsigned int zend_uint;  
typedef unsigned long zend_ulong;  
typedef unsigned short zend_ushort;

From the above code, we can see that _zvalue_value is the key part that actually saves data. Weakly typed variable declaration through union

Question 2. How does the Zend engine identify and store multiple data types in PHP?

_zval_struct.type stores the real type of a variable, and you can choose how to get the value of zvalue_value based on the type.

type value list (Zend/zend.h):

#define IS_NULL     0  
#define IS_LONG     1  
#define IS_DOUBLE   2  
#define IS_BOOL     3  
#define IS_ARRAY    4  
#define IS_OBJECT   5  
#define IS_STRING   6  
#define IS_RESOURCE 7  
#define IS_CONSTANT 8  
#define IS_CONSTANT_ARRAY   9

Let’s look at a simple example:

<?php  
$a = 1;  
//此时zval.type = IS_LONG,那么zval.value就去取lval.  
$a = array();  
//此时zval.type = IS_ARRAY,那么zval.value就去取ht.

The most complex of these, and what is often used in developing third-party extensions, is "resources" Type".

In PHP, any variable that does not belong to PHP's built-in variable type will be treated as a resource and saved.

For example: database handle, open file handle, open socket handle.

The resource type will use lval, which is an integer indicator at this time. Then PHP will query the corresponding resource in a resource list built in PHP based on this indicator.

It is precisely because of ZE's processing method that PHP implements weak types, and for ZE, it always faces the same type zval.