What are the PHP constants?

php constants include: 1. Logical constants, used to represent true and false values; 2. System constants, which are some constants predefined by the system; 3. Numeric constants, used to represent numerical values, which can be integers or floats. Points; 4. String constants, used to represent a string of characters, including single quotes and double quotes; 5. Array constants, used to represent a set of values; 6. Class constants, used only within the class, with Used to represent specific attributes of a class.

#PHP constants are identifiers that cannot be changed during program execution. In PHP, constants are defined using the define() function. The format for defining constants is as follows: define("constant name", "constant value", whether it is case-sensitive).

PHP constants have the following commonly used types:

1. Logical constants: Logical constants are used to represent true and false values, including TRUE and FALSE. The sample code is as follows:

define("IS_TRUE",true);
define("IS_FALSE",false);

2. System constants: System constants are some constants predefined by the system and can be used directly, for example:

define("PHP_VERSION",phpversion());//获取PHP版本号
define("PHP_OS",PHP_OS);//获取操作系统类型

3. Numeric constants: Numeric constants are used to represent numerical values. , which can be an integer or a floating point number. The sample code is as follows:

define("PI",3.14);//定义圆周率
define("MAX_SIZE",1024);//定义最大尺寸

4. String constant: String constant is used to represent a string of characters, including single quotes and double quotes. The sample code is as follows:

define("GREETING","Hello,world!");//定义问候语

5. Array constant: Array constant is used to represent a collection of values. The sample code is as follows:

define("FRUITS",['apple','banana','orange']);//定义水果常量

6. Class constants: Class constants are attributes of the class and can only be used inside the class to represent specific attributes of the class. The sample code is as follows:

classMyClass{
constMAX_VALUE=100;//定义最大值常量
}

The above are some common PHP constant types, which can be defined and used as needed. When using constants, you can directly use the constant name to access the value of the constant without using the $ symbol. For example:

echoIS_TRUE;//输出TRUE
echoPHP_VERSION;//输出PHP版本号

It should be noted that once a constant is defined, it cannot be modified or deleted, and the constant name is not case-sensitive (unless the third parameter is set to true when defining). The scope of constants is global and can be accessed throughout the script .

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