What is the use of php define

In PHP, the define() function is used to define a constant, the syntax is "define($name, $value, $case_insensitive)"; the parameter "$case_insensitive" is used to specify whether the constant is case-sensitive, the default It is case-sensitive. If the value is set to true, it means that it is not case-sensitive. Once a constant is defined, it cannot be redefined or undefined.

The operating environment of this tutorial: windows7 system, PHP8 version, DELL G3 computer

define is a built-in function in PHP, used to define a constant.

A constant is a simple identifier. This value cannot be changed during script execution (except for so-called magic constants, which are not constants). Constants are case-sensitive by default. Normally constant identifiers are always uppercase.

Constants are similar to variables. The difference is that after setting, the value of the constant cannot be changed. There is no need for the dollar sign ($) at the beginning. The scope does not affect access to the constant. The value of the constant only Can be a string or a number.

define() function defines constants

Legal constant names can only start with letters and underscores, and can be followed by letters, numbers, or underscores of any length . 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

<?php
header("Content-type:text/html;charset=utf-8");
define(&#39;WebSite&#39;, &#39;php中文网&#39;);
echo WebSite.&#39;<br>&#39;;
?>

Extended knowledge:

const keyword defines constants

After php5.3.0, you can use the const keyword to define constants outside the class definition. In previous versions, the const keyword can only be used in the class ( class).

Syntax:

const 常量名 = 常量值;

Example:

<?php
const FOO = &#39;BAR&#39;;
echo FOO.&#39;<br>&#39;;
?>

The difference between define() and const:

const is defined at compile time Constants, and 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(&#39;BIT_5&#39;, 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(&#39;BIT_&#39; . $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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