深入php define()函数以及defined()函数的用法详解_php技巧

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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
案例1
Define a case-sensitive constant:
指定一个常量(区分大小写):

<?phpdefine("GREETING","Hello you! How are you today?");echo constant("GREETING");?>

The output of the code above will be:
上述代码将输出下面的结果:

Hello you! How are you today?

Example 2
案例2
Define a case-insensitive constant:
指定一个常量(不区分大小写):

<?phpdefine("GREETING","Hello you! How are you today?",TRUE);echo constant("greeting");?>

The output of the code above will be:
上述代码将输出下面的结果:

Hello you! How are you today?

The defined() function checks whether a constant exists.
defined()函数的作用是:检查一个常量是否存在。

Returns TRUE if the constant exists, or FALSE otherwise.
如果该常量存在,则返回True;如果不存在,则返回False。

Syntax
语法

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
案例

<?phpdefine("GREETING","Hello you! How are you today?");echo defined("GREETING");?> 

The output of the code above will be:
上述代码将输出下面的结果:

1

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