PHP Constants: Const vs. Define

Defining constants in PHP, const is more suitable for constant definitions inside classes, and define() is more flexible and suitable for global or dynamic definitions. 1. const is a language structure, and must be a compile-time constant expression when defined, which is suitable for class or global namespaces; define() is a function, and the value can be the result of runtime calculations. 2. Const is affected by the namespace, while constants defined by define() are globally visible by default. 3. The const has a clear structure and good IDE support, which is suitable for object-oriented design; define() has high flexibility but may have higher maintenance costs. 4. define() supports runtime condition judgment and dynamic definition, but const does not. Therefore, class-related constants use const first, and global or environment-related constants use define().

There are two common methods to define constants in PHP: const and define() . Although they all do the task, there are differences in usage scenarios, flexibility and readability. Simply put, const is more suitable for constant definitions inside a class, while define() is more flexible and suitable for global or dynamic definitions.

1. Syntax and usage scenarios

const is a language structure, while define() is a function.

• const is usually used in classes or in global namespaces and must be a compile-time constant expression when defined (not a value calculated at runtime).
• define() can be called anywhere, and the value can be the result of the runtime calculation.

example:

 // Usage of const class Status {
    const ACTIVE = 1;
}

// Usage of define define('MAX_ATTEMPTS', 5);

Applicable suggestions:

• If you define fixed values such as status codes, type identifiers, etc. in the class, use const to be clearer.
• If you need to dynamically define constants based on your environment (such as based on configuration files), it is more appropriate to use define() .

2. Scope differences

const and define() are handled differently in scope.

• const is affected by namespace and is also applicable to class inside.
• Constants defined by define() are global by default and are not restricted by namespace.

example:

 namespace App;

const VERSION = '1.0'; // define('APP_VERSION', '2.0'); // globally visible echo VERSION; // Correct: Output App\VERSION
echo APP_VERSION; // Correct: Global constants

Note:

• If you use a namespace, the access to const needs to be prefixed with the namespace.
• Constants defined by define() are accessible throughout the script, so naming conflicts should be avoided when used.

3. Performance and readability

From a performance perspective, the two are not much different, but readability and maintenance are worth considering.

• const is more suitable for object-oriented design, with clear structure and good IDE support.
• define() is flexible but easily scattered throughout the code, and may be more costly to maintain.

suggestion:

• Class-related constants use const first.
• define() can be considered for global configuration and environment-related constants.
• Use one method in a unified way to reduce confusion.

4. Dynamic definition and conditional judgment

define() supports defining constants at runtime, while const must determine the value at compile time.

example:

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

The above code cannot be implemented using const because const does not support conditional judgment.

Applicable scenarios:

• Need to check whether a constant has been defined at runtime → define()
• Values depend on function return or configuration read → define()

Basically that's it. Both have their own advantages, just choose according to the specific scenario. It is the most common practice to use const in the class, define() globally or dynamically define it.

The above is the detailed content of PHP Constants: Const vs. Define. For more information, please follow other related articles on the PHP Chinese website!