What is the difference between constants and variables in php?

#What are constants in PHP?

The so-called constant is a quantity that cannot be changed. Once a PHP constant is defined, it cannot be changed or undefined. This is the official explanation and the most authoritative explanation. PHP constants are usually used to store data that does not change and is not expected to change. The data can only be data of four scalar data types: integer (integer), floating point (decimal), string, Boolean (true) /false).

Naming rules for PHP constants

The naming rules for PHP constants are similar to the naming rules for PHP variables but not exactly the same.

● PHP constant name must be composed of letters, underscores, and numbers;

● It cannot start with a number;

● Whether to be case-sensitive or not is specified when defining ;

● It is recommended to know the meaning when naming (you will know what it means when you see it), use English words as constant names, and it is recommended that the letters are all capitalized (such as: PI);

The difference between php constants and variables

Different definition forms: php variables have no defined keywords or functions and are defined when assigning values. PHP constants must be defined using the define() function.

Different names: PHP variable names need to be modified with a dollar sign ($), but PHP constant names do not. It is recommended that PHP variable names be lowercase. When they consist of multiple words, the first letter of the first word should be lowercase, and the first letter of the remaining words should be uppercase. It is recommended that PHP constant names be in all uppercase letters.

Different degrees of variability: the values ​​stored in PHP variables can be changed, while the values ​​stored in PHP constants cannot be changed.

Scope is not used: PHP variables are defined outside all functions and have global scope; when defined within functions, they have local scope. PHP constants are generally defined outside all functions and have super-global scope.

The data type range is different: PHP variables can be of any data type, while PHP constants can only be scalar types (integer, float/double, string, boolean).

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