Detailed explanation of the difference between const and define in PHP

##Available when defining constants in PHP What is the difference between const and define?

1. Const is used to define class member variables. Once defined, its value cannot be changed. define defines global constants that can be accessed anywhere.

2. define cannot be defined in a class, but const must be defined in a class, and variables defined by const must be accessed through class name::variable name.

3. Const constants cannot be defined in conditional statements.

4. const uses an ordinary constant name (static scalar), and define can use any expression as the name.

5. const is always case-sensitive, but define() can define case-insensitive constants through the third parameter.

6. Using const is simple and easy to read. It is a language structure in itself, and define is a method. Using const to define is much faster than define at compile time.

If you define a constant in a class, you cannot use define, but use const, as in the following example:

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PHP Tutorial"

<?php
//在类外面通常这样定义常量
define("PHP","111cn.net");
class MyClass
{
    //常量的值将始终保持不变。在定义和使用常量的时候不需要使用$符号
    const constant = &#39;constant value&#39;;

    function showConstant() {
        echo  self::constant . "<br>";
    }
}

echo MyClass::constant . "<br>";

$classname = "MyClass";
echo $classname::constant . "<br>"; // PHP 5.3.0之后

$class = new MyClass();
$class->showConstant();
echo $class::constant."<br>"; // PHP 5.3.0之后
//print_r(get_defined_constants());  //可以用get_defined_constants()获取所有定义的常量
?>

Generally define defines constants outside the class, const defines constants within the class, and const must be accessed through class name::variable name. However, php5.3 and above support defining constants outside of classes through const. See the following. This is OK:

<?php
   const a = "abcdef";
   echo a;
?>

I won’t go into the basic knowledge about constants here. In addition to the above, define and const are other things. Difference (from the Internet):

1.const cannot define constants in conditional statements, but define is possible, as follows:

<?php
if(1){
   const a = &#39;java&#39;;
 }    
echo a;  //必错
?>

2.const uses an ordinary constant name , define can take an expression as the name

<?phpconst  FOO = &#39;PHP&#39;; 
for ($i = 0; $i < 32; ++$i) { 
    define(&#39;PHP_&#39; . $i, 1 << $i); 
} 
?>

3.const can only accept static scalars, while define can take any expression.

<?php
const PHP = 1 << 5; // 错误
define(&#39;PHP&#39;, 1 << 5); // 正确 
?>

4.const itself is a language structure. And define is a function. So using const is much faster.

The two have something in common: both cannot be reassigned.

The following content is excerpted from Rotted_Pencil's blog post: The difference between defining constants in PHP, define() vs. const

Preface

Read it again on Stackoverflow today I came across a very interesting article, so I translated it and picked it up. The article was written by NikiC, one of the PHP development members, and its authority is unquestionable

Text

In PHP5.3, there are two ways to define constants:

1. Use the const keyword

2. Use the define() method

const FOO = ‘BAR’; 
define(‘FOO’,’BAR’);

The fundamental difference between the two methods is that const will define a constant when the code is compiled, while define will A constant is defined when the code is running. This causes const to have the following disadvantages:

const cannot be used in conditional statements. If you want to define a global variable, const must be at the outermost level of the entire code:

if (...) {    
    const FOO = 'BAR';    // 无效的
}
// but
if (...) {
   define('FOO', 'BAR'); // 有效的
}

You may ask why I want to do this? One of the most common examples is when you are checking whether a constant has been defined:

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

const can only be used to declare variables (such as numbers, strings, or true, false, null, FILE), and define() can also accept expressions. However, after PHP5.6 const can also accept constant expressions:

const BIT_5 = 1 << 5;    // 在PHP5.6之后有效，之前无效
define(&#39;BIT_5&#39;, 1 << 5); // 一直有效

const constant names can only use straightforward text, while define() allows you to use any expression to name them. Constant naming. This allows us to do the following:

for ($i = 0; $i < 32; ++$i) {
    define(&#39;BIT_&#39; . $i, 1 << $i);
}

const-defined constants are case-sensitive, but define allows you to turn off its case-sensitivity by setting its third parameter to true:

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

The above are the points you need to pay attention to. So now I will explain the following, why I personally always use const without involving the above situations:

const is more readable and beautiful.

const defines constants under the current namespace by default, and using define requires you to specify the full path of the entire namespace:

namespace A\B\C; 
// 如果要定义常量 A\B\C\FOO: 
const FOO = ‘BAR’; 
define(‘A\B\C\FOO’, ‘BAR’);

Since PHP5.6, const arrays can also be defined. is a constant. Define currently does not support this function, but this function will be implemented in PHP7:

const FOO = [1, 2, 3];    // 在PHP 5.6中有效 
define(‘FOO’, [1, 2, 3]); // 在PHP 5.6无效, 在PHP 7.0有效

Because const is executed during compilation, it is faster than define.

Especially when using define to define a large number of constants, PHP will run very slowly. People even invented things like apc_load_constantshide to avoid this problem

Compared with define, const can double the efficiency of defining constants (on a development machine configured with XDebug, this difference will be even greater). But in terms of query time, there is no difference between the two (because both use the same query table)

The last thing to note is that const can be used in class and interface, while define is Those who cannot do this:

class Foo {
    const BAR = 2; // 有效
}
class Baz {
    define('QUX', 2); // 无效
}

Summary

Unless you need to use expressions or define constants in conditional statements, otherwise you'd better use const just for the simple readability of the code!

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