The difference between constants and variables in php

The difference between constants and variables

1. There is no dollar sign ($) in front of the constant

　2. Constants can only be defined using the define() function, not assignment statements

　3. Constants can be defined and accessed anywhere regardless of the rules of variable scope

4. Once a constant is defined, it cannot be redefined or undefined

5. The value of a constant can only be a scalar

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Constant

The broad concept of "constant" is: 'a quantity that does not change' (For example: a quantity that will not be modified by the program when the computer program is running; a certain quantity in a mathematical function, such as the radius and diameter of each specific circle; the acceleration of gravity near the ground in physics; the acceleration of gravity in a vacuum The value of the speed of light; the respective masses of different particles) In other words, constants, in terms of computer technology, serve hardware, software, and programming languages, but they are not a concept introduced specifically for hardware, software, and programming languages. Constants can be divided into different types. For example, 25, 0, and -8 are integer constants, 6.8, and -7.89 are real constants, and 'a' and 'b' are character constants. Constants can generally be judged from their literal form. Such constants are called literal constants or direct constants.

When discussing how the value of a function changes (becomes larger or smaller) with the relationship between independent variables, there are often one or more constants in the function. People focus on studying the rules of these changes, often It is easy to find the correct reflection in the brain of the laws of the development of things.

The popular metaphor for 'constant' - "just like a mountain that cannot be changed easily" (of course, the gravitational acceleration on the earth will also change when the sea is dry and the rocks are shattered; and the constant people use is that which does not take a very long time) A quantity that basically does not change for a period of time. The speed of light in a vacuum state is a constant and cannot be changed in theory. It is a constant)

Variable

Variables come from mathematics and are abstract concepts in computer language that can store calculation results or represent values. Variables can be accessed by variable name. In imperative languages, variables are usually mutable; in purely functional languages, such as Haskell, variables may be immutable. In some languages, variables may be explicitly referred to as abstractions with storage space that can represent mutable state (such as in Java and Visual Basic); but other languages ​​may use other concepts (such as objects in C) to refer to such variables. Abstract, but not strictly define the precise denotation of "variable".

Variables are useful because they allow you to give a short, easy-to-remember name to each piece of data you plan to use in your program. Variables can save data entered by the user when the program is running (such as using the InputBox function to display a dialog box on the screen, and then saving the text entered by the user into the variable), the results of specific operations, and a piece of data to be displayed on the form, etc. . In short, variables are simple tools for tracking almost any type of information.

If there is no value assigned after the variable is declared, the compiler will automatically prompt and assign a default value.

A variable is a convenient placeholder used to reference a computer memory address, which can store program information that can be changed when the Script is running. For example, you can create a variable called Click Count to store the number of times a user clicks on an object on a web page. Using variables does not require knowing the address of the variable in computer memory. You can view or change the value of the variable by referencing the variable by its name. There is only one basic data type in VB Script, namely Variant, so the data type of all variables is Variant.

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