Summary of knowledge about constants in PHP

This article mainly introduces relevant information about constant knowledge in PHP. Friends who need it can refer to it

Constants in PHP are divided into custom constants and system constants

Characteristics of constants:

1. After setting, the value of the constant cannot be changed

2. The constant name does not require a dollar sign at the beginning ($)

3. Scope does not affect access to constants

4. Constant values ​​can only be strings or numbers
   

Get the value of a constant:

• Use the constant name to get the value directly;

• Use ==constant( )== function, it has the same effect as directly using the constant name to output, but the function can dynamically output different constants, so it must be flexible and convenient to use.
  

defined() function: Determine whether a constant has been defined

define("PI",3.14);

$is1 = defined('PI');
$is2 = defined('PI1');

var_dump($is1); // boolean true
var_dump($is2); // boolean false

##1. Custom constant

In PHP, use the ==define()== function to define a constant

define("PI",3.14);
echo constant('PI');

define("GREETING","Hello world!",TRUE);
echo constant('greeting');

2. System constants

Common system constants are:

1. __FILE__: PHP program file name. It can help us get the physical location of the current file on the server.

2. __LINE__: The number of lines in the PHP program file. It can tell us which line the current code is on.

3. PHP_VERSION: The version number of the current parser. It can tell us the version number of the current PHP parser, and we can know in advance whether our PHP code can be parsed by the PHP parser.

4. PHP_OS: The name of the operating system that executes the current PHP version. It can tell us the name of the operating system used by the server and we can optimize our code according to that operating system.

echo __FILE__; // E:\Web\Project\_wamp\demo.php
echo __LINE__; // 3
echo PHP_VERSION; // 5.6.25
echo PHP_OS; // WINNT

The above is the entire content of this article, I hope it will be helpful to everyone's study.

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