Detailed explanation of the difference and use of const and static in php

This article will explain the difference and use of const and static in php.

First of all, regarding const, you can only modify member attributes within a PHP class, but not methods, as follows:

class Test{ 
    const PATH = 'c/';//修饰常量 
    const function test(){//这种方法是错误的，const无法修饰方法 
    } 
}

There is no need to add permission modification fields when using const, but when calling const Modifying constants requires skills, because constants belong to the entire class, not to a certain object, so you need to use the class name plus the domain name modifier when calling, for example:

echo Test::PATH; 
//也可以使用下面的方法调用 
$test = new Test(); 
echo  $test::PATH;

But in the class Problems will arise when calling internally. How to get the current class name or current object name inside the class (php can get the class name based on the object - reflection). PHP provides us with two special keywords, such as:

class Test{ 
    const PATH = 'c/';//修饰常量 
    public function ass(){ 
        echo $this::PATH.&#39;<br />&#39;; 
        echo self::PATH.&#39;<br />&#39;; 
    } 
}

Where self (without $) represents the current class name, $this represents the current object. In this way, we can call const modified constants inside the class.

Next let’s talk about static

Although const is very useful, the value cannot be modified once it is defined. But sometimes we need an attribute that belongs to the class, but it can be modified. value, such as counting how many objects this class has constructed. This is the time to use static. When static modifies the member variables of the class, its usage is basically the same as const. The difference is that static-modified attributes can add permissions. (const can also be added in PHP 7.1, but not in the past), and the value of statically modified member variables can be modified.

But when a member method is statically modified, the usage method changes a little. Inside the class, inside the statically modified method body, any ordinary member variables of the current class cannot be accessed. To be precise, they cannot be used. The keyword $this can only use static member variables and class constants of the current class.

class Test{ 
    static public $name='asd'; 
    const π = 3.1415926; 
    public $age = 10; 
    public static function lenth($r){ 
        //echo $this->age;//会发生错误，不能访问普通的成员变量 
        echo Test::$name; 
        return $r*2*self::π;     
    } 
    public static function area($r){ 
        return $r*$r*self::π; 
    }    
}

In the process of using static, the method called is different from the variable modified by static. It can be called through the class name or the object name, such as:

echo &#39;周长：&#39;. Test::lenth(1).&#39;<br/>&#39;; 
echo &#39;面积：&#39;.Test::area(1).&#39;<br/>&#39;; 
$per = new Test(); 
echo $per->lenth(3);

The above Both calling methods are legal.

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