The constant value is immutable, and the variable value is variable. Const is defined with define() or const and cannot be modified and is used to store fixed information such as configuration and status code; variables are defined with $ and can be modified multiple times, suitable for dynamic data such as user input. Global access for constants does not require global, and global or $GLOBALS is required for variable scope limitations. Selection by: Whether the value changes, such as constants used in database information, and variables used in user login status. Note: Constant names are all capitalized, value-limited scalar types, PHP8.1 supports arrays, variable names cannot start with numbers, avoid mutable variables, use defined() to check whether they exist.

Constants and variables are two different ways of data storage in PHP. Although they look similar, they have obvious differences in their uses and behavior. If you have written PHP code, you should encounter these two concepts frequently. Simply put: the value of a variable can be changed, but once the value of a constant is defined, it cannot be changed .

Basic usage of Constants

Constants are used to store unchanged data, such as configuration information, project paths, API keys, etc. It cannot be modified or redefined after definition.

Use define() or const to define:

 define('SITE_NAME', 'MySite');
// Or const SITE_URL = 'https://example.com';

The main difference between the two is:

• define() is a function that can be defined at runtime;
• const is a language structure that can only be defined at compile time and is usually used in classes.

Accessing constants does not require a $ sign:

 echo SITE_NAME; // Output MySite

Constant scopes are global by default and there is no need to declare global additionally.

Variables flexibility

Variables are more flexible and are suitable for frequently changing data, such as user input, loop counters, temporary results, etc.

Defining variables is simple:

 $name = 'John';
$age = 25;

$ must be added before the variable name, and the variable can be modified multiple times during the program running:

 $count = 10;
$count = 5;
echo $count; // Output 15

The scope of variables is limited by functions, classes or files. If you want to access global variables inside the function, you need to use the global keyword or $GLOBALS array.

Constant vs variable: When to use which one?

When choosing a constant or a variable, it depends on whether this value will change during operation.

? Constants are used:

• Storage project configuration (such as database connection information)
• Define fixed status codes (such as STATUS_ACTIVE , STATUS_INACTIVE )
• Business rules that do not change with the environment

? Not suitable for using constants:

• User input content
• Dynamically generated data
• Values that need to be updated multiple times

For example: If you want to represent a user's login status, the variable is more appropriate at this time; and if you want to define "maximum number of login attempts", you can use constants.

Notes and common mistakes

Some small details that novices tend to ignore:

• Constant names are generally in full capitalization, and multiple words are separated by underscores (conventional)
• Constants cannot be defined repeatedly, otherwise an error will be reported.
• The value of a constant can only be of scalar type (int, float, string, bool). PHP 8.1 has started to support arrays
• Variable naming should not start with numbers and should not contain special characters.
• Try to avoid using mutable variables (such as $$var), which will make the code difficult to maintain

To check whether the constant exists, you can use defined() function:

 if (!defined('MY_CONSTANT')) {
    define('MY_CONSTANT', true);
}

Basically that's it. Understanding the difference between constants and variables will allow you to write clearer and more stable PHP code. If you use the right place, you will get twice the result with half the effort.

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