What is the definition and difference between constants and variables in php

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.

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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.

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 through 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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