What are magic methods and magic constants in php

In PHP, magic methods refer to methods starting with two underscores "__", such as "__call()", "__get()", "__set()", etc.; magic constants refer to methods that will 8 constants that change with the change of their code location, such as "__LINE__", "__FILE__", etc.

The operating environment of this tutorial: windows7 system, PHP7.1 version, DELL G3 computer

Magic method (Magic methods)

In PHP, methods starting with two underscores __ are called magic methods. These methods play a pivotal role in PHP. Magic methods include:

• __construct(), class constructor
• __destruct(), class destructor
• __call(), when calling an inaccessible method in the object, call
• __callStatic(), when calling an inaccessible method in static mode Call
• __get(), call
• __set() when obtaining a member variable of a class, call # when setting a member variable of a class
• ##__isset(), called when isset() or empty() is called on an inaccessible property
• __unset(), called when unset() is called on an inaccessible property.
• __sleep(), when executing serialize(), this function will be called first
• __wakeup(), executed unserialize(), this function will be called first
• __toString(), the response method when the class is treated as a string
• __invoke(), the response method when calling an object by calling a function
• __set_state(), when calling var_export() when exporting a class, this static method will be called.
• __clone(), when the object copy is completed, call

##__construct() and __destruct() Constructors and destructors should be familiar, they are called when objects are created and destroyed. For example, we need to open a file, open it when the object is created, and close it when the object dies

<?php 
class FileRead
{
    protected $handle = NULL;

    function __construct(){
        $this->handle = fopen(...);
    }

    function __destruct(){
        fclose($this->handle);
    }
}
?>

These two methods can be expanded when inheriting, for example:

<?php 
class TmpFileRead extends FileRead
{
    function __construct(){
        parent::__construct();
    }

    function __destruct(){
        parent::__destruct();
    }
}
?>

__call () and __callStatic()These two methods are called when an inaccessible method is called in the object, and the latter is a static method. These two methods may be used in variable method (Variable functions) calls.

<?php
class MethodTest 
{
    public function __call ($name, $arguments) {
        echo "Calling object method &#39;$name&#39; ". implode(&#39;, &#39;, $arguments). "\n";
    }

    public static function __callStatic ($name, $arguments) {
        echo "Calling static method &#39;$name&#39; ". implode(&#39;, &#39;, $arguments). "\n";
    }
}

$obj = new MethodTest;
$obj->runTest(&#39;in object context&#39;);
MethodTest::runTest(&#39;in static context&#39;);
?>

__get(), __set(), __isset() and __unset()These two functions are called when getting/set member variables of a class. For example, we save the object variables in another array instead of the member variables of the object itself

<?php 
class MethodTest
{
    private $data = array();

    public function __set($name, $value){
        $this->data[$name] = $value;
    }

    public function __get($name){
        if(array_key_exists($name, $this->data))
            return $this->data[$name];
        return NULL;
    }

    public function __isset($name){
        return isset($this->data[$name])
    }

    public function unset($name){
        unset($this->data[$name]);
    }
}
?>

##__sleep()

and__wakeup()When we execute serialize()

and

unserialize(), these two functions will be called first. For example, when we serialize an object, the object has a database link. If we want to restore the link state during deserialization, we can restore the link by reconstructing these two functions. An example is as follows:

<?php
class Connection 
{
    protected $link;
    private $server, $username, $password, $db;

    public function __construct($server, $username, $password, $db)
    {
        $this->server = $server;
        $this->username = $username;
        $this->password = $password;
        $this->db = $db;
        $this->connect();
    }

    private function connect()
    {
        $this->link = mysql_connect($this->server, $this->username, $this->password);
        mysql_select_db($this->db, $this->link);
    }

    public function __sleep()
    {
        return array(&#39;server&#39;, &#39;username&#39;, &#39;password&#39;, &#39;db&#39;);
    }

    public function __wakeup()
    {
        $this->connect();
    }
}
?>

__toString()

The response method when the object is treated as a string. For example, use echo $obj;

to output an object

<?php
// Declare a simple class
class TestClass
{
    public function __toString() {
        return &#39;this is a object&#39;;
    }
}

$class = new TestClass();
echo $class;
?>

This method can only return a string, and an exception cannot be thrown in this method, otherwise a fatal error will occur.

__invoke()

The response method when calling an object by calling a function. As follows

<?php
class CallableClass 
{
    function __invoke() {
        echo &#39;this is a object&#39;;
    }
}
$obj = new CallableClass;
var_dump(is_callable($obj));
?>

__set_state()

Call var_export()

This static method will be called when exporting a class.

<?php
class A
{
    public $var1;
    public $var2;

    public static function __set_state ($an_array) {
        $obj = new A;
        $obj->var1 = $an_array[&#39;var1&#39;];
        $obj->var2 = $an_array[&#39;var2&#39;];
        return $obj;
    }
}

$a = new A;
$a->var1 = 5;
$a->var2 = &#39;foo&#39;;
var_dump(var_export($a));
?>

__clone()

Called when the object copy is completed. For example, in the singleton mode implementation method mentioned in Detailed Explanation of Design Patterns and PHP Implementation: Singleton Mode, this function is used to prevent objects from being cloned.

<?php 
public class Singleton {
    private static $_instance = NULL;

    // 私有构造方法 
    private function __construct() {}

    public static function getInstance() {
        if (is_null(self::$_instance)) {
            self::$_instance = new Singleton();
        }
        return self::$_instance;
    }

    // 防止克隆实例
    public function __clone(){
        die(&#39;Clone is not allowed.&#39; . E_USER_ERROR);
    }
}
?>

Magic constants

Most of the constants in PHP are unchanged, but there are 8 constants that change with their location. These 8 constants change depending on the code location. These 8 constants are called 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
• __FUNCTION__
, the function name
• __CLASS__
, the name of the class
• __TRAIT__
, Trait name
• __METHOD__
, class method name
• __NAMESPACE__
, name of current namespace
• These magic constants are often used to obtain current environment information or record logs.

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