PHP development basic tutorial constants

Concept

Constant can be understood as: a long-lasting value. After the constant value is defined, it cannot be changed anywhere else in the script.

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1. Composition of constants

The writing form of constants is define(constant name, constant)

Note:

• Constant names can be lowercase, but usually uppercase

• Constant names can be without quotation marks, but usually with quotation marks.

• When calling a constant in a string, it must be outside the quotation marks

• It is recommended that only letters and underscores be used for constant names

Let’s define and call a constant:

Example:

<?php
//定义一个常量
define('Name','PHP.com');
//正确的调用方式
echo '我们是'.Name;
echo '<br/>';
//错误的调用方式
echo '我们是Name';
?>

Next, let’s verify several statements about constants

Example ：

<?php
//其实可以小写，但是不好区分出来，所以习惯上大家都大写
define('age',27);
echo age;
//常量可以在外面不加引号
define(A,'安徽省');
echo A;
//只能用于标量，用于其他会报错，例如数组
define('HF',array(1,2,3));
echo HF;
?>

Note 1: The constant definition is not quoted and can be output normally, but PHP will remind us that the definition is not standardized. Here Ntice

can Eliminate it by modifying the PHP configuration file. If you are interested, you can search and try to solve it yourself.

Note 2: The seven data types mentioned earlier can also be divided into three major categories

Scalar data types: Boolean, Integer, floating point, string

Composite data type: array, object

Special data type: NUll, resource type, callback Function

Note 3: After a constant is defined, it is a global variable by default and can be used anywhere in the entire running script.

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2. Some built-in constants

The system also prepares some built-in constants for us. These constants are specified. Let’s get familiar with a few first. There are more system constants that we have studied in the previous volume. After getting started, we will slowly add and learn them.

##__FILE__The path of the current file on the server__FUNCTIOIN__Current function namePHP_VERSIONCurrent PHP Version__TRAIT____DIR____NAMESPACE__

Example:

<?php
//输出当前所在行
echo __LINE__;
echo "<br/>";
//输出当前文件所在服务器的路径
echo __FILE__;
echo "<br/>";
//输出当前PHP运行的操作系统
echo PHP_OS;
?>

Note: For the other few, you can try to output them. There are a few that you haven’t learned yet. You can learn them in later chapters. Detailed introduction

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3. Use the defined() function to create a security mechanism

Knowledge here, Just understanding for now. After learning the function, you can look at this part of the code again.

defined() Let’s learn this usage, mainly to prevent others from bypassing the security check file.

Function: defined (constant)
Function: Pass the constant after the parentheses of the function. If the constant is defined, it returns true, otherwise it returns false

Assumption:

We have a set of online e-mall software that requires payment. Checking whether payment is made is done by checking the software authorization. The file version.php has the function of checking authorization. We have stipulated in the software that there is no authorization check file. version.php cannot use this software. All code includes version.php. And in order to prevent someone from piracy, I can also encrypt the version.php code.

We have two files:

• There is a version number, version statement and authorization statement in the middle of one file. The file name is version.php

• #A file has specific business functions. For example: user registration, login, etc., the file name is users.php

What should we do? ——That is to say, if the file version.php is not included, the code after users.php will be executed.

Let’s experiment:

1.version.php file

The code is as follows:

<?php
//此处是检查是否是否授权的业务部份代码xxxx
define('AUTH',true);
//略过模拟代码xxx行
?>

2.ser.php

The code is as follows:

<?php
//尝试将include 'version.php'这一行代码注释后再执行看看，对比结果
include 'version.php';
if(!defined('AUTH')){
    echo '非法！非法！你尝试跳过授权文件';
    exit;
}

The results show that version.php must be included, otherwise the following echo 'User Registration' will not be displayed;

Note:

Function: include('pass in file path and file name')
Function: The function of this function is to pass in the file with the specified path and let PHP include it for execution

Inclde will be explained in detail later

Next Section

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##Constant name	Description
__LINE__	The current line
__CLASS__	Current class name
##__METHOD__	The operating system PHP runs on
##Trait name, added in php5.4
The directory where the file is located
The name of the current namespace (case sensitive)