What is the difference between define and define

The differences between define and defined are: 1. Define is used to define a constant and it cannot be changed or canceled after it is defined; 2. Define checks whether the constant is defined and returns true if it exists. Returns false if it does not exist.

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define and defined The difference

define

define is used to define a constant. The constant represents the global scope, so it can be used directly in the script without considering the scope. accessible from anywhere. But one thing to note is that once a constant is defined, it cannot be changed or undefined

Example:

define("path","root/www/web")

where root/www/web is the value of the constant

defined

#defined is used to detect whether the constant is defined. If the constant exists, it returns true. If it does not exist, it returns false.

Example :

defined('APP_PATH') or define('APP_PATH', dirname($_SERVER['SCRIPT_FILENAME']) . DS);

The meaning of the above code is that if app_path is defined, it will return true. If it is not defined, it will execute the code after or

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