Can php change the value of a constant?

php cannot change the value of a constant. A constant is a quantity that cannot be changed. Once defined, a constant cannot be modified, redefined or undefined; it is usually used to store data that does not change and is not expected to change. The data type can be integer, floating point, Strings, booleans, and arrays.

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

Corresponding to variables are constants, so-called constants It is a quantity that cannot be changed. Once a constant in PHP is defined, it cannot be modified or undefined.

PHP constants are usually used to store data that does not change and is not expected to change. The data can only be data of four scalar data types: integer, floating point, string, and Boolean. However, Starting from PHP7, constants support array types.

Compared with variables, constants have the following characteristics:

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

• Constants can only be defined using define() and const;

• The scope of constants is global;

• Once defined, constants cannot be redefined or undefined.

How to define constants?

Please refer to the article "How to set constants in php"

How to determine whether a constant exists?

To determine whether a constant has been defined, you can use the defined() function. The syntax format of the function is as follows:

defined(string $name)

where $name is the name of the constant to be determined. The function will return a bool type value. If the constant is defined, it returns true, if it is not defined, it returns false. The sample code is as follows:

<?php
header("Content-type:text/html;charset=utf-8");
define(&#39;WebSite&#39;, &#39;php中文网&#39;);
const url = &#39;https://www.php.cn/&#39;;
var_dump(defined(&#39;WebSite&#39;));
var_dump(defined(&#39;url&#39;));
?>

Output results:

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