Declaration, assignment and output of PHP constants

* 1. Three elements of constants:

* 1.1 Once created, it cannot be deleted or modified;

* 1.2 Do not add the '$' character in the header

* 1.3 Global scope, use

directly without declaration in the function * 2. Create:

* 2.1 Function: define('CONST_NAME', expression);

* 2.2 Keyword: const contName = static scalar, can be used in classes, cannot be used in functions

* 2.3 Constant names: usually only use uppercase letters and underlines, such as: ROOT_PATH

* 3. Access:

* 3.1 Keyword echo direct access: echo const_name;

* 3.2 Function: constant('constant name');

* 3.3 Function: get_defined_constants (): Get all constants

* 4. Detection: defined('constant name')

//Create constants

define('SITE_NAME','Peter Zhu的博客'); //函数define创建
const COUNTRY ='中国';  //关键字const创建

//Access constants

echo SITE_NAME,COUNTRY;  //关键字echo 访问
echo &#39;<hr>&#39;;
echo constant(&#39;SITE_NAME&#39;); //函数constant访问
echo constant(&#39;COUNTRY&#39;);  //函数constant访问
echo &#39;<hr><pre class="brush:php;toolbar:false">&#39;;

//Get all constants defined by the system, including user-defined

print_r(get_defined_constants()); 
echo &#39;<hr>&#39;;

//Detect constants

echo defined('SITE_NAME') ? '已定义' : '未定义'; //检测常量是否已声明