A deep understanding of the Variables in PHP principles (Variables inside PHP)

Maybe you know, maybe you don't know, PHP is a weak type, dynamic scripting language. The so-called weak type means that PHP does not strictly verify the variable type (strictly speaking, PHP is a medium-strong language, which will be described in future articles ), when declaring a variable, you do not need to specify the type of data it stores:

 
 

  
  
<?php
  
  
  $var = 1; //int
  
  
  $var = "laruence"; //string
  
  
  $var = 1.0002; //float
  
  
  $var = array(); // array
  
  
  $var = new Exception('error'); //object;
 
 

Dynamic language, that is, the PHP language structure can be changed at runtime. For example, we require a function definition file at runtime, which leads to dynamic changes to the function table of the language.
The so-called scripting language means that PHP is not run independently. To run php, we need a PHP parser:

 
 

  
  
  /usr/bin/php -f example.ph
 
 

As I have mentioned in the previous article, PHP is executed through Zend engine (Ze, Zend engine), And Ze is written in C. Everyone knows that C is a strongly typed language, that is, all variables in C can only save one type of data when declared to the final destruction. So how does PHP implement weak types based on ze?

In PHP, all variables are saved using a structure-zval. In Zend/Zend. H, we can see the definition of zval:

 
 

  
  
  typedef struct _zval_struct {
  
  
    zvalue_value value;
  
  
    zend_uint refcount;
  
  
    zend_uchar type;
  
  
    zend_uchar is_ref;
  
  
  } zval;
  
  
 
 
 

Zvalue_value is the key part of data storage. Now it's time to reveal the answer. How does PHP implement weak types based on ze? Because zvalue_value is a union ),

 
 

  
  
typedef union _zvalue_value {
  
  
    long lval;
  
  
    double dval;
  
  
    struct {
  
  
        char *val;
  
  
        int len;
  
  
    } str;
  
  
    HashTable *ht;
  
  
    zend_object_value obj;
  
  
} zvalue_value 
   
 
    
;
 
   
 
 

So how does this structure store various types of PHP?
Common variable types in PHP Include:

 
 

  
  
1. Integer/floating point/long integer/bool value, etc.
  
  
2. String
  
  
3. array/associated array
  
  
4. Object
  
  
5. Resources
  
  
 
 
 

PHP stores the real type of a Variable Based on the Type field in zval, and then selects how to obtain the zvalue_value value based on the type, such as for integer and bool values:

 
 

  
  
Zval. type = IS_LONG; // integer
  
  
Zval. type = IS_BOOL; // Boolean
 
 

Take zval. value. lval. For the bool value, lval ε (0 | 1 );
For dual precision or float, the zval. Value dval is obtained.
If it is a string, then:

 
 

  
  
   zval.type = IS_STRIN
 
 

At this time, it will take:
Zval. value. Str
This is also a structure that contains the length of the C-shard string and the string.

For arrays and objects, the type corresponds to IS_ARRAY and IS_OBJECT, and the corresponding values are zval. value. ht and obj respectively.

Resources are special. in PHP, resources are special variables. Any variables that do not belong to the built-in variable type in PHP will be considered as resource sources for storage. For example, database handle, open file handle, and so on. For resources:

 
 

  
  
   type = IS_RESOURC
 
 

At this time, zval will be retrieved. value. lval. At this time, lval is an integer indicator. Then, PHP will query the corresponding resources in the PHP built-in resource list based on this indicator (this part of content, I will introduce it in a separate article later). Currently, you only need to know the lval at this time as it corresponds to the offset value of the resource linked list.

 
 

  
  
 ZEND_FETCH_RESOURCE(con, type, zval *, default, resource_name, resource_type
);
 
 

Using this mechanism, PHP implements a weak type, because for ZE, it will always be faced with the same type, that is, zval.
Ps: The team will go out to building tomorrow. I thought I should write something for my blog reader before leaving to spend the weekend. Today, let's start with something simple. Next time, I will introduce the PHP variables, scopes, and the copy on write and change on write mechanisms of variables ....