In-depth explanation of the php define() function and the usage of the defined() function_PHP Tutorial

The define() function defines a constant.
define()函数的作用是：定义一个常量。
Constants are much like variables, except for the following differences:
常量[constant]与变量[variable]有很多相似的地方，因此，很容易混淆；下面，我们列举一下常量[constant]与变量[variable]之间的不同点：

•A constant's value cannot be changed after it is set
一个常量值在指定之后就不可以更改；
•Constant names do not need a leading dollar sign ($)
设置常量时，不需要在前面加上“$”符号；
•Constants can be accessed regardless of scope
常量可以被所有范围的域访问；
•Constant values can only be strings and numbers
常量的值只能是“字符串[string]”和“数字[number]”；

Syntax
语法

Parameter 参数	Description 描述
name	Required. Specifies the name of the constant 必要参数。指定常量的名称
value	Required. Specifies the value of the constant 必要参数。指定常量的值
case_insensitive	Optional. Specifies whether the constant name should be case-insensitive. If set to TRUE, the constant will be case-insensitive. Default is FALSE (case-sensitive) 可选参数。指定常量的名称是否是不区分大小写的[case-insensitive]。如果设置为True，则不区分字母大小写；如果设置为False，则区分字母大小写。默认值是：False

Example 1
Case 1
Define a case-sensitive constant:
Specify a constant (case-sensitive):

The output of the code above will be:
The above code will output the following results:

Example 2
Case 2
Define a case-insensitive constant:
Specify a constant (case-insensitive):

The output of the code above will be:
The above code will output the following result:

The defined() function checks whether a constant exists. The function of the defined() function is to check whether a constant exists.
Returns TRUE if the constant exists, or FALSE otherwise.

If the constant exists, it returns True; if it does not exist, it returns False.


Syntax
Syntax

Parameter Parameter	Description Parameter 参数 Description 描述 name Required. Specifies the name of the constant to check 必要参数。指定常量对象的名称 Description
name	Required. Specifies the name of the constant to checkRequired parameter. Specify the name of the constant object

Example
Case

The code is as follows:

The output of the code above will be:
The above code will output the following result: The code is as follows: