Getting Started with PHP Constants_PHP Tutorial

Constants, like variables, are used to temporarily store a value, but constants differ from variables in many ways.

Constant:
1. Data cannot be changed during program execution, and the scope of constants is global.
2. The naming of constants is similar to that of variables, but without the dollar sign "$". A valid constant name begins with a letter or an underscore, followed by any number of letters, numbers, or underscores.
3. Generally, constants in PHP are and are divided into system constants and custom constants.

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We just talked about it in general, and we will introduce this knowledge later.

1. __FILE__ Default constant refers to the PHP program file name and path;
2. __LINE__ Default constant refers to the number of lines of the PHP program;
3. __CLASS__ The name of the class;

: Define a constant through the define() function,

The syntax format is: bool define ( string $name , mixed $value [, bool $case_insensitive = false ] )

name: Specify the name of the constant (uppercase).
value: Specify the value of the constant. (Only scalars and null are allowed. The type of a scalar is an integer, a float, a string, or a boolean. It is also possible to define the type of a constant value as a resource, but this is not recommended and may lead to unknown situations. )
insensitive: Specifies whether the constant name is case-sensitive. If set to true, it is case-insensitive; if set to false, it is case-sensitive. If this parameter is not set, the default value of false is taken to be case-sensitive.

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Let’s take a look at an example:

<?php
//定义一个大小写敏感的常量：
define("CONSTANT", "Hello world");
echo CONSTANT; // 输出：Hello world
echo Constant; // 报错

//定义一个大小写不敏感的常量：
define("GREETING", "Hello you", true);
echo GREETING; // 输出：Hello you
echo Greeting; // 输出：Hello you
?>

After understanding this example, it is better to write constants in uppercase letters in the future.

At the same time, we also need to pay attention to the printing of constants:

1. When you want to print a constant, you cannot enclose the constant in quotation marks, like the example above;

2. Use the connection operator to print constants

<?php
define("CONSTANT", "Hello world");
echo 'I say：' . CONSTANT; 
?>

Look here, the past constant is not very simple, but there are many ways to use it. For example, the most commonly used one is:

Define a constant to determine illegal calls between pages. Specifically, when we make the home page of a web page, we often separate the home page into two files, the header and the foot. At this time, we need to prevent someone from illegally operating, directly Call these two files. At this time, we will directly add this piece of code to these two files

<?php
//防止恶意调用
if (!defined('CMS_DH')) {
	exit('Access Defined!');
}
?>

Add the following code to the home page (head and foot files must be imported on the home page. The specific introduction method will be discussed later. If you are interested, you can import it via Baidu Google php file yourself)

<?php
//定义个常量，用来授权调用头部和脚部2个文件
define('CMS_DH',true);
?>

The above is just a small example. I will talk about it later, or dig it out yourself.

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