Detailed explanation of the difference between PHP constants define and const

In our previous article, we introduced you to examples of the use of define in PHP. I believe you all have a certain understanding of define. Today we will introduce to you a detailed explanation of the difference between PHP constants define and const. !

A constant is a simple identifier. This value cannot be changed during the execution of the script (except for the so-called magic constants, which are not actually 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 constant

2. Constants can only be defined through the define() function, not assignment statements

3. Constants can ignore the role of variables Domain can be defined and accessed anywhere

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

5. Constant The value 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 to define 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 can use expressions 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 Take any expression

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

6、const定义的常量时大小写敏感，而define可以通过第三个参数（为true表示大小写不敏感）来指定大小写是否敏感。

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

总结：

本文通过实例为大家更为直观的了解php常量define与const的区别详解，希望对你的工作有所帮助！

相关推荐：

php中define的使用详解

php define常量详解

php define的第二个参数使用方法

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