Discuss the php define() function and detailed explanation of the use of 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
define(name,value,case_insensitive)

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):

Copy code The code is as follows:

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

Copy Code The code is as follows:

Hello you! How are you today?

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

Copy code The code is as follows :

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

Copy the code The code is as follows:

Hello you! How are you today?

The defined() function checks whether a constant exists.
defined() function The function is: 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

Copy code The code is as follows:

defined(name)

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

Example
Case

Copy code The code is as follows:

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