Understand php constants in one minute

There are many quantities that do not change in PHP. They are called constants. This time we will learn about the definition, use, naming and differences of constants from variables. Follow the editor to learn.

1. Concept

Constants are used to temporarily store values ​​and will only exist when we execute the program.

2. Define and use

define('常量名称',常量值)

or

define("常量名称",常量值)

Constant naming:

• and Variables can also only contain letters, numbers, and underscores, and must start with letters or underscores, not numbers;

• Constant names are usually uppercase;

• Customized constants are strictly case-sensitive.

3. The difference between constants and variables

• ##There is no dollar sign in front of the constant (

  $ ), there is a $ symbol in front of the variable;

• constants cannot be assigned through assignment statements, and can only be defined using the

  define() function;

• Like c, constants can be defined and used anywhere regardless of variable scope;

• After a constant is defined, it cannot be It cannot be redefined or undefined;

• The value of a constant can only be a Boolean type, integer, floating point or string type;

4. Predefined constants

Predefined constants: Constants that have been defined in the PHP kernel and can be used directly.

Magic constant: It is a predefined constant starting with

__. The value is different depending on the location of the code, so it is called a magic constant.

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