What does underscore mean in php

One underscore in php represents private variables and private methods, and two underscores are PHP built-in variables. For example, PHP stipulates that methods starting with two underscores are reserved as magic methods, so it is recommended that you have the best function name. Do not start with __ unless you are overloading an existing magic method.

The operating environment of this article: Windows7 system, PHP7.1 version, DELL G3 computer

What does the underline mean in php?

The meaning of variables starting with an underscore in PHP

Naming rules

Adding one is private

Adding two is generally the system default , predefined by the system, the so-called:

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"Magic method" and "Magic constant" ”

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★PHP constants starting and ending with double underscores are “magic constants”:

__LINE__The current line number in the file.

__FILE__The full path and file name of the file.

__DIR__The directory where the file is located. If used within an included file, returns the directory where the included file is located. It is equivalent to dirname(__FILE__). Unless it is the root directory, the directory name does not include the trailing slash

Note: The above content comes from "PHP Chinese Manual->Language Reference->Constant->Magic Constant".

From php5 and later versions, php classes can use magic methods.

PHP stipulates that methods starting with two underscores (__) are reserved as magic methods, so it is recommended that your function name should not start with __ unless it is to overload an existing magic method.

The magic methods in PHP are: __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 the class and their parent class

__get( $property ) When an undefined property is called, this method Will be triggered, the parameter passed is the property name being accessed

__set( $property, $value ) When assigning a value to an undefined property, this method will be triggered, the parameter passed is the property being set Name and value

The undeclared here includes attributes whose access control is protected and private (that is, attributes that do not have permission to access) when called using an object.

2, __isset, __unset

__isset( $property ) This method is called when the isset() function is called on an undefined property

__unset( $property ) This method is called when the unset() function is called on an undefined property.

The same as the __get method and __set method. There is no declaration here. When calling with an object, the access control is protected, private. Attributes (that is, attributes that do not have permission to access)

3. __call

__call( $method, $arg_array ) This method is called when calling an undefined 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 the method. If it does not exist in the parent class, call the __call() method of this class. If this class If the __call() method does not exist in the parent class, go to the __call() method

4 and __autoload

__autoload function, which will automatically appear when trying to use a class that has not yet been defined. transfer. By calling this function, the scripting engine has a last chance to load the required classes before PHP fails with an error.

If you want to define a global autoloading class, you must use the spl_autoload_register() method to register the processing class to the PHP standard library:

<?php  
class Loader  
{  
static function autoload_class($class_name)  
{  
//寻找正确的$class_name类，并引入，没有则抛出异常  
}  
}  
  
/** 
*  设置对象的自动载入 
*  spl_autoload_register — Register given function as __autoload() implementation 
*/  
spl_autoload_register(array(&#39;Loader&#39;, &#39;autoload_class&#39;));  
  
$a = new Test();//Test没用require就实例化，实现自动加载，很多框架就用这种方法自动加载类  
  
?>

Note: Exceptions thrown in the __autoload function It cannot be caught by the catch statement block and cause a fatal error, so it should be caught in the function itself.

5, __construct, __destruct

__construct constructor method, this method is called when an object is created. The advantage of using this method compared to PHP4 is that the constructor method can have a unique name. No matter what the name of the class it is in is. In this way, when you change the name of the class, you do not need to change the name of the constructor method

__destruct destructor method. PHP will remove the object from memory before it is destroyed. Before clearing) call this method. By default, PHP only releases the memory occupied by object properties and destroys object-related resources. The destructor allows you to execute arbitrary code to clear memory after using an object. When PHP decides that your script is no longer associated with the object, the destructor will be called.

In the namespace of a function, this will happen when the function returns.

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 pointing to the object. Usually assign the variable to NULL or call unset.

6, __clone

Object assignment in PHP5 uses reference assignment. If you want to copy an object, you need to use the clone method. When calling this method, the object will automatically call the __clone magic method. If the object is copied, some initialization operations need to be performed. , can be implemented in the __clone method.

7, __toString

__toString method is automatically called when converting an object into a string, such as when using echo to print the object.

If the class does not implement this method, the object cannot be printed through echo, otherwise it will display: Catchable fatal error: Object of class test could not be converted to string in

This method must return a string.

Before PHP 5.2.0, the __toString method can only take effect when used in conjunction with echo() or print(). After PHP 5.2.0, it can take effect in any string environment (for example, through printf(), using the %s modifier), but cannot be used in non-string environments (such as using the %d modifier). From PHP 5.2.0, if an object that does not define the __toString method is converted to a string, an E_RECOVERABLE_ERROR error will be reported.

8, __sleep, __wakeup

__sleep When serializing, use

__wakeup. When deserializing, call

serialize() to check the class Is there a function with the magic name __sleep. If so, the 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.

The purpose of using __sleep is to close any database connections the object may have, submit pending data, or perform similar cleanup tasks. In addition, this function is also useful if you have very large objects that do not need to be stored completely.

In contrast, unserialize() checks for the existence of a function with the magic name __wakeup. This function can reconstruct any resources the object may have, if present.

The purpose of using __wakeup is to reestablish any database connections that may have been lost during serialization and to handle other reinitialization tasks.

9, __set_state

When var_export() is called, this static method will be called (valid since PHP 5.1.0).

The only parameter of this method is an array, which contains class properties arranged in the format of array('property' => value, ...).

10. __invoke

When trying to call an object by calling a function, the __invoke method will be automatically called.

Valid for PHP5.3.0 or above

11. __callStatic

It works similar to the __call() magic method. __callStatic() is to handle static method calls.

Valid for PHP5.3.0 and above

PHP has indeed strengthened the definition of the __callStatic() method; it must be public and must be declared static. Likewise, the __call() magic method must be defined as public, as must all other magic methods.

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