What is the way to define constants in php

How to define constants in php: 1. Use the define() function to define, the syntax "define (constant name, constant value, whether case sensitive)"; 2. Use the const keyword to define, the syntax "const Constant name = constant value;".

The operating environment of this tutorial: windows7 system, PHP7.1 version, DELL G3 computer

How to define constants in php

• define() function

• const keyword

Compared with variables, Constants have the following characteristics:

• There is no dollar sign ($) in front of the constant;

• Constant can only be defined with define() and const ;

• The scope of constants is global;

• Once a constant is defined, it cannot be redefined or undefined.

define() function defines constants

Legal constant names can only start with letters and underscores, and can be followed by letters of any length, Numbers or underscores. In PHP, you can define a constant through the define() function. The syntax of the define() function is as follows:

define($name, $value , $case_insensitive)

• $name Required parameters, constant name (need to use double quotes Or wrapped in single quotes)

• $value Required parameter, the value of the constant

• $case_insensitive Optional parameter, specifies whether it is case sensitive, The default is case-sensitive. If set to true, it means that it is not case-sensitive

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const keyword definition constant

Syntax:

const 常量名 = 常量值;

Example:

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##The difference between define() and const:

const defines constants at compile time, while the define() method defines constants at runtime.

const cannot be used in if statements, defne() can be used in if statements.

if(...) { const FOO = 'BAR';//错误 } if(...) { define('FOO', 'BAR');//正确 }

define() A common scenario is to first determine whether the constant has been defined before defining the constant:

if(defined('FOO)) { define('FOO', 'BAR') }

const When defining a constant, the value can only be a static scalar (number, string, true , false, null), and the define() method can use the value of any expression as the value of a constant. Starting from PHP 5.6, const also allows expressions to be used as the value of constants.

const BIT_5 = 1 << 5; //PHP5.6后支持，之前的PHP版本不支持 define('BIT_5', 1 << 5);// 所有PHP版本都支持const 只允许简单的常量名，而define()可以把任何表达式的值用作常量名

for ($i = 0; $i < 32; $i++) { define('BIT_' . $i, 1 << $i); }

const The constant names defined are case-sensitive, and case-insensitive constants can be defined by passing true to the third parameter of the define() method.

define('FOO', 'BAR', true); echo FOO; //BAR echo foo; //BAR

Listed above are some shortcomings or inflexibility of const compared to define(). Let’s take a look at why I personally recommend using const instead of define() to define constants (unless you need to Define constants in the scenarios listed above).

const has better readability, const is a language structure rather than a function, and is consistent with the form of defining class constants in a class.

const defines constants in the current namespace, and define() must pass the complete namespace name when defining to achieve similar effects:

namespace A\B\C; //To define the constant A\B\C\FOO: const FOO = 'BAR'; define('A\B\C\FOO', 'BAR');

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