Detailed explanation of how to use php magic methods_php skills

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

• __construct(), class constructor
• __destruct(), destructor of class
• __call(), called when calling an inaccessible method in an object
• __callStatic(), called when calling an inaccessible method in static mode
• __get(), called when obtaining a member variable of a class
• __set(), called when setting member variables 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(), when executing 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(), this static method will be called when var_export() is called to export the class.
• __clone(), called when the object copy is completed
  

__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

<&#63;php 
class FileRead
{
 protected $handle = NULL;

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

 function __destruct(){
  fclose($this->handle);
 }
}
&#63;>

These two methods can be extended when inherited, for example:

<&#63;php 
class TmpFileRead extends FileRead
{
 function __construct(){
  parent::__construct();
 }

 function __destruct(){
  parent::__destruct();
 }
}
&#63;>

__call() and __callStatic()

These two methods will be 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.

<&#63;php
class MethodTest 
{
 public function __call ($name, $arguments) {
  echo "Calling object method '$name' ". implode(', ', $arguments). "\n";
 }

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

$obj = new MethodTest;
$obj->runTest('in object context');
MethodTest::runTest('in static context');
&#63;>

__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

<&#63;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]);
 }
}
&#63;>

__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. Examples are as follows:

<&#63;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('server', 'username', 'password', 'db');
 }

 public function __wakeup()
 {
  $this->connect();
 }
}
&#63;>

__toString()

Response method when the object is treated as a string. For example, use echo $obj; to output an object

<&#63;php
// Declare a simple class
class TestClass
{
 public function __toString() {
  return 'this is a object';
 }
}

$class = new TestClass();
echo $class;
&#63;>

This method can only return a string, and exceptions 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

<&#63;php
class CallableClass 
{
 function __invoke() {
  echo 'this is a object';
 }
}
$obj = new CallableClass;
var_dump(is_callable($obj));
&#63;>

__set_state()

This static method will be called when var_export() is called to export a class.

<&#63;php
class A
{
 public $var1;
 public $var2;

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

$a = new A;
$a->var1 = 5;
$a->var2 = 'foo';
var_dump(var_export($a));
&#63;>

__clone()

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

<&#63;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('Clone is not allowed.' . E_USER_ERROR);
 }
}
&#63;>

Magic constants

Most of the constants in PHP are unchanged, but there are 8 constants that will change as their location in the code changes. 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__, function name
• __CLASS__, the name of the class
• __TRAIT__, the name of Trait
• __METHOD__, the method name of the class
• __NAMESPACE__, the name of the current namespace
  

These magic constants are often used to obtain current environment information or record logs.

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