Definition of constants in PHP

1. Custom Constants
* Must be defined using the define () function
* After definition, the value cannot be changed.
* The constant name is used directly. You cannot add $ s in front of a variable.
For example: define ("PI", 3.14); defines a constant
$ Area = PI * R; Calculate the area of the circle
Define ("URL", "http://www.ybldcc.com ");
Echo "my website is:". URL;
2 system constants:
FILE: php program FILE name
LINE: Number of PHP Program Files
PHP_VERSION: version number of the current parser
PHP_ OS: name of the operating system for executing the current PHP version
Author: "ITeamsky-Yang Bo's technical space"