The meaning of variables starting with underscores in PHP

Named rules Add one as private Adding two is generally the system default , System predefined , The so-called ： ===================== “ Magic methods ” And “ magic constant ” ===================== *PHP Constants that start and end with double underscores are “ magic constant ”：

__LINE__ The current line number in the file .

__FILE__ The full path and filename of the file .

__DIR__ Directory of files . If used in included files , Returns the directory where the included files are located . It is equivalent to dirname(__FILE__). Unless it's the root directory , Otherwise, the name in the directory does not include the trailing slash

notes ： The above is from “PHP Chinese Manual -> Language reference -> Constant -> magic constant ”.

from php5 Later versions ,php Class can use magic methods .

php Specify two underscores （__） The first methods are reserved as magic methods , So it's better not to use function names __ start , Except for the purpose of overloading existing magic methods .

PHP There are many magic methods in :__construct, __destruct , __call, __callStatic,__get, __set, __isset, __unset , __sleep, __wakeup, __toString, __set_state, __clone, __autoload

1、__get、__set

These two methods are designed for properties that are not declared in classes and their parent classes

__get( $property ) When calling an undefined property , This method will be triggered , The parameter passed is the name of the property being accessed

__set( property, value ) When assigning a value to an undefined property , This method will be triggered , The parameters passed are the name and value of the set property

There is no declaration here, including when using object calls , Access control is proteced,private Properties of （ That is, there is no access to the property ）.

2、__isset、__unset

__isset( $property ) When called on an undefined property isset() Function

__unset( $property ) When called on an undefined property unset() Function

And __get Methods and __set In the same way , There is no declaration here, including when using object calls , Access control is proteced,private Properties of （ That is, there is no access to the property ）

3、__call

__call( method, arg_array ) When you call an undefined method, you call this method

The undefined methods here include methods that do not have permission to access ; If the method does not exist, go to the parent class to find this method , If the parent class does not exist, call the __call() Fang ? Law , If it does not exist in this class __call() Method is to find the __call() Method

4、__autoload

__autoload function , It will automatically call when trying to use a class that has not been defined . By calling this function , The script engine is in PHP There was a last chance to load the required classes before the error failed .

If you want to define a global automatic loading class , You have to use spl_autoload_register() Method registers the processing class to PHP Standard library ：

<?php    
class Loader    
{    
static function autoload_class($class_name)    
{    
// Look for the right $class_name class , And introduce , If not, throw an exception     
}    
}    
    
/**  
*   Set the automatic loading of objects   
*  spl_autoload_register — Register given function as __autoload() implementation  
*/    
spl_autoload_register(array('Loader', 'autoload_class'));    
    
$a = new Test();//Test useless require Is instantiated , Implement automatic loading , Many frameworks use this method to automatically load classes     
    
?> 

Copy

Be careful : stay __autoload An exception thrown in a function cannot be catch Statement blocks capture and cause fatal errors , So capture should be done in the function itself .

5、__construct、__destruct

__construct Construction method , This method is called when an object is created , be relative to PHP4 The advantage of using this method is ： You can give a constructor a unique name , Whatever the name of the class it's in . So when you change the name of a class , There is no need to change the name of the constructor

__destruct destructor ,PHP Before the object is destroyed （ That is, before clearing from memory ） Call this method . By default ,PHP Just release the memory occupied by the object properties and destroy the resources related to the object , Destructors allow you to clear memory by executing arbitrary code after using an object . When PHP When you decide that your script is no longer related to an object , The destructor will be called .

In the namespace of a function , This happens to the function return When .

For global variables , This happens at the end of the script .

If you want to explicitly destroy an object , You can assign any other value to the variable that points to the object . Usually the variable is assigned to NULL Or call unset.

6、__clone

PHP5 Object assignments in are reference assignments used , If you want to copy an object, you need to use clone Method , When this method is called, the object will automatically call __clone Magic methods , If you need to perform some initialization operation in object replication , Can be in __clone Method realization .

7、__toString

__toString Method is called automatically when converting an object to a string , For example, use echo When printing objects .

If the class does not implement this method , They can't get through echo Print object , Otherwise it will show ：Catchable fatal error: Object of class test could not be converted to string in This method must return a string .

stay PHP 5.2.0 Before ,__toString Methods can only be used in combination echo() or print() when To take effect .PHP 5.2.0 after , Can take effect in any string environment （ For example, through printf(), Use %s Modifier ）, but Cannot be used in non string environments （ If you use %d Modifier ）. from PHP 5.2.0, If an undefined __toString Object of method Convert to string , Will report a E_RECOVERABLE_ERROR error .

8、__sleep、__wakeup

__sleep When serializing, use

__wakeup Called when deserializing

serialize() Check for magic names in the class __sleep Function of . If so , This function will run before any serialization . It clears the object and should return an array containing the names of all variables in the object that should be serialized .

Use __sleep The purpose of is to close any database connection that the object may have , Submit pending data or perform similar cleanup tasks . Besides , This function is also useful if you have very large objects and don't need to store them completely .

By contraries ,unserialize() Check for magic names __wakeup The existence of a function of . If there is , This function rebuilds any resources an object might have .

Use __wakeup The purpose of is to rebuild any database connections that may be lost in serialization and handle other reinitialization tasks .

9、__set_state

When calling var_export() when , This static Method will be called （ since PHP 5.1.0 Effective ）.

The only argument to this method is an array , It includes pressing array(’property’ => value, …) Class properties of formatting .

10、__invoke

When trying to call an object as a function ,__invoke Method will be called automatically .

PHP5.3.0 The above version is valid

11、__callStatic

It works in a way similar to __call() Magic methods ,__callStatic() To handle static method calls ,

PHP5.3.0 The above version is valid PHP It really strengthens the right __callStatic() Method definition ; It has to be public , And must be declared static . Again ,__call() Magic methods have to be defined as public , All other magic methods must be like this .

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