Detailed introduction to the difference and usage of self and this in php

The difference between this and self keywords in PHP:

1. self represents the class, $this represents the object
2. Can Where $this is used, self must be used. Where self can be used, $this
3 cannot be used. $this cannot be used in static methods. Static methods are accessed by classes.

Usage of this and self keywords in PHP:

self is the class name that refers to the static class, and $this refers to the instance of the non-static class name.

static properties and methods, only static properties and methods can be accessed, and non-static properties and methods cannot be accessed by class.

Because when static properties and methods are created, there may not be any instances of this class that can be called.

static properties have only one copy in memory and are shared by all instances.

Use the self:: keyword to access static members of the current class.

All instances of a class share static properties in the class.

In other words, even if there are multiple instances in the memory, there is only one copy of the static attributes.

In the following example, a counter $count attribute is set, and private and static modifications are set.

In this way, the outside world cannot directly access the $count attribute. As a result of the program running, we also see that multiple instances are using the same static $count attribute.

<?php    
class user  
{    
   private static $count = 0 ; //记录所有用户的登录情况.    
   public function __construct() {    
       self::$count = self::$count + 1;    
   }    
   public function getCount() {      
       return self::$count;    
   }    
   public function __destruct() {    
       self::$count = self::$count - 1;    
   }    
}    
$user1 = new user();    
$user2 = new user();    
$user3 = new user();    
echo "now here have " . $user1->getCount() . " user";    
echo "<br />";    
unset($user3);    
echo "now here have " . $user1->getCount() . " user";    
?>

Static properties are directly called

Static properties can be used directly without instantiation. They are used before the class is created. Can be used directly.

The method used is: class name::static property name

<?php    
class Math  
{    
   public static $pi = 3.14;    
}    
// 求一个半径3的园的面积。    
$r = 3;    
echo "半径是 $r 的面积是<br />";    
echo Math::$pi * $r * $r;    
echo "<br /><br />";    
//这里我觉得 3.14 不够精确，我把它设置的更精确。    
Math::$pi = 3.141592653589793;    
echo "半径是 $r 的面积是<br />";    
echo Math::$pi * $r * $r;      
?>

The class is not created, and the static properties can be used directly.

Static method

Static methods can be used directly without the class being instantiated.

The method used is class name::static method name

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