What is the difference between two ways to define PHP constants: define and const?

This article will take you to understand PHP constants, compare the two definition methods of PHP constants, and talk about their differences. I hope it will be helpful to everyone!

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.

You can use the define() function to define 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 classes. Once a constant is defined, it cannot be changed or undefined.

Constants can only contain scalar data (boolean, integer, float and string). Resource constants can be defined, but should be avoided as they can cause unpredictable results.

You can get the value of a constant simply by specifying its name. Unlike variables, you should not add the $ sign in front of a constant. If the constant name is dynamic, you can also use the function constant() to get the value of the constant. Use get_defined_contstants() to get a list of all defined constants.

The differences between constants and variables are as follows:

1. There is no dollar sign ($) in front of the constants

2. Constants can only pass define() Function definition, but not through assignment statements

3. Constants can be defined and accessed anywhere regardless of the scope of the variable

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

5. The value of a constant can only be a scalar

<?php
// 以下代码在 PHP 5.3.0 后可以正常工作
const USERNAME = &#39;zhouguowei&#39;;
echo USERNAME;
echo constant("USERNAME");
 
const ZHOUUSERNAME = &#39;zhouguowei2222222222&#39;;
 
define(&#39;MYUSERNAME&#39;,&#39;zhouguowei1111111&#39;);
echo "<pre class="brush:php;toolbar:false">";
print_r(get_defined_constants());  
?>

Q: When defining constants in PHP, what is the difference between const and define?

Answer: Using const makes the code simple and easy to read. const itself is a language structure, and define is a function. In addition, const is much faster than define during compilation.

1. Const is used for the definition of class member variables. Once defined, it cannot be modified. Define cannot be used to define class member variables, but can be used for global constants.

2. Const can be used in classes, but define cannot.

3. Const cannot define constants in conditional statements.

<?php
if (...){
const FOO = &#39;BAR&#39;;    // 无效的invalid
}
if (...) {
define(&#39;FOO&#39;, &#39;BAR&#39;); // 有效的valid
}
?>

4. Const uses ordinary constant names, define Expressions can be used as names

<?php
const  FOO = &#39;BAR&#39;;
for ($i = 0; $i < 32; ++$i) {
define(&#39;BIT_&#39; . $i, 1 << $i);
}
?>

5. const can only accept static scalars, while define can use any expression

<?php
const BIT_5 = 1 << 5;    // 无效的invalid
define(&#39;BIT_5&#39;, 1 << 5); // 有效的valid
?>

6. Constants defined by const are case-sensitive, while define You can specify whether case sensitivity is through the third parameter (true indicates case insensitivity).

<?php
define(&#39;FOO&#39;, &#39;BAR&#39;, true);
echo FOO; // BAR
echo foo; // BAR
?>

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