What are the ways to define php constants?

What are the definition methods of PHP constants

Constant

Features: Cannot be modified, cannot Destroyed and cannot be deleted. After declaring a constant, the constant can be used anywhere on the page

Methods for declaring constants: const, define, static

Naming convention: and The variables are the same, but all capitalized

define: define is a function, which cannot be defined in an object, but can be defined and used in a class;

define('CL',10);
echo CL;
//判断常量是否存在
if(defined('CL')){
    echo 'ture';        
}else{
    echo 'false';
}

static: Static constant

Static constant: It is a variable that can be accessed using constant syntax, which is::, and can be accessed without instantiation

Note: static is only initialized once and called recursively will not be repeatedly initialized

 //定义静态常量    
 class Person{     
     public static $a = "呵呵";     
     public static function say(){      
         echo "我丢：".self::$a."<br>";     
         }    
      }    
    //输出静态属性    
    echo Person::$a."<br>";    
    //调用静态方法    
    Person::say();    
    //修改静态属性    
    Person::$a = "我靠";    
    echo Person::$a."<br>";

const: const is a language structure, which is faster than define when compiling. It is a global constant that can be defined in an object and used in a class. It can be understood as Class constants

 class CL    {            
  //定义常量       
   const CLS = '常量值';       
   function a() {                
   //调用常量方法            
       echo self::CLS;            
  }    
}    
(new CL)->a();

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