What is the relationship between php constants and variables

During the execution of the program, the value stored in a variable can be changed at any time, but the value stored in a constant cannot be changed.

Variables are used to store temporary data information. When a variable is defined, the system will automatically allocate a storage space for the variable to store the variable's value. We can assign a value to a variable when we define it. If we need to change the value of the variable, we only need to assign it again. Temporary data information or processing procedures can be stored in variables.

Constants are used to store data information that does not change frequently. You can assign a value to a constant when you define it. During the entire execution period of the program, this assignment is valid and the constant cannot be assigned again.

Let’s first look at the difference between PHP constants and variables from the definition of the two :

Constant: In PHP we use the define() function to define constants

Variables: All variables in PHP start with $, followed by the name of the variable, and assign a value to the variable.

From the definition of PHP constants and variables, we can get:

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

2. Constants can only use define() Function definition, not through assignment statements;

Let’s look at the difference between PHP constants and variables after they are defined:

Constant: Once a constant is defined, it cannot be Redefine or undefine.

Variable: Variable refers to the amount that the value can change during the execution of the program. It is opposite to the constant we are talking about.

In this way we can get the following result:

Constants cannot be changed as long as they are defined, but variables can be changed.

Look at the values ​​of constants and variables again:

Constant: The value of a constant can only be a scalar, that is, it can be an integer, floating point, Boolean, null string etc., it cannot be an array object resource, and variables can be all data types of PHP (eight data types of PHP).

Let’s take a look at the scope of PHP constants and variables:

Scope of constants: No matter where they are defined, constants can be called directly without scope.

Scope of variables: The variable is valid in that scope in which it is defined. For example, a variable defined in a function is only valid in the function.

Summary:

Through the above content, we can summarize the differences between PHP constants and variables:

1. There is no constant in front of it Dollar sign ($)

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

The above content is for reference only.

For more related content, please visit the PHP Chinese website: PHP Video Tutorial

The above is the detailed content of What is the relationship between php constants and variables. For more information, please follow other related articles on the PHP Chinese website!