Use const first because it parses at compile time, has better performance and supports namespaces; 2. When you need to define constants in conditions, functions, or use dynamic names, you must use define(); 3. Only const can be used in classes to define constants; 4. define() can dynamically define expressions and complete namespace strings at runtime; 5. Once both are defined, they cannot be modified, but define() can avoid repeated definitions through defined(), while const cannot be checked; 6. The const name must be literal and does not support variable interpolation. Therefore, const is suitable for fixed and clear constants, and define() is suitable for scenarios that require runtime logic or dynamic naming. The selection should be based on specific requirements.

In PHP, defining constants is a common requirement in development, such as storing configuration items, status codes, or global invariant values. For a long time, PHP has provided two main ways to declare constants: define() function and const keyword. Although their goals are consistent - creating immutable values - there are important differences in usage scenarios, syntax limitations, and operational mechanisms. This article will analyze the difference between define() and const in depth to help you understand when to use which one.

1. Basic syntax and usage method

define() : Functional definition, runtime execution

 define('PI', 3.14159);
define('SITE_NAME', 'My Website');

• define() is a function that can be called dynamically at runtime.
• The first parameter is the constant name (string), and the second is the value.
• Expressions are supported as values (starting from PHP 5.6).

const : language structure, compile-time parsing

 const PI = 3.14159;
const SITE_NAME = 'My Website';

• const is a language structure, not a function, and cannot be called dynamically.
• The value must be determined at compile time and can only be used in top-level scopes or classes.
• The value must be a static scalar or scalar expression (PHP 5.6 supports simple operations).

2. Scope and definition location

This is one of the most significant differences between the two.

define() : Flexible scope control

• It can be called anywhere , including conditional statements, function inside, and loop.

 if (true) {
    define('DEBUG_MODE', true);
}

function setup() {
    define('APP_PATH', '/var/www');
}
setup();

? It is very convenient to define constants after dynamic judgment during runtime.

const : limited by compile-time parsing

• Only available in the top level of the file or in the class/interface/traits .
• It cannot appear in runtime structures such as conditions, functions, and loops.

 if (true) {
    const DEBUG = true; // ? Fatal error: const cannot be used in the condition}

 function foo() {
    const BAR = 1; // ? Error}

? Suitable for well-defined, fixed global constants or class constants.

3. Naming flexibility: dynamic vs static names

define() supports dynamic constant names

 $env = 'prod';
define("API_KEY_$env", 'abc123');
echo API_KEY_prod; // Output abc123

This is useful when configuring multiple environment constants.

The const name must be literal

 const "API_KEY_$env" = 'abc123'; // ? Syntax error

Variable interpolation or dynamic naming is not supported.

4. Performance and parsing timing

• const is parsed at the compilation stage , so it accesses slightly faster and is engine-optimized earlier.
• define() is executed only at runtime , with a slight overhead of function calling.

Although the performance difference is extremely small (microsecond level), const is better in high-frequency calling scenarios.

5. Use in the class

In a class, you can only use const , and you cannot use define() to define class constants.

 class Status {
    const PENDING = 0;
    const APPROVED = 1;
}

echo Status::PENDING; // 0

Note: Class constants are accessed via :: , not $this-> .

define() defines global constants, even if written in class methods:

 class Config {
    public function set() {
        define('MAX_SIZE', 1024); // Global constant, not part of the class}
}

6. Variability and repeated definition

Once defined, neither can be modified or redefined (otherwise an error will be reported).

 define('NAME', 'Alice');
define('NAME', 'Bob'); // ? Fatal error: constants are defined const NAME = 'Alice';
const NAME = 'Bob'; // ? Compilation error

But you can avoid repeated definitions by defined() :

 if (!defined('NAME')) {
    define('NAME', 'Alice');
}

const cannot be checked like this, and it must be ensured that it does not repeat.

7. Namespace behavior differences

const follows namespace rules

 namespace App;

const PI = 3.14;
// The actual constant is named App\PI

Pay attention to the namespace when accessing:

 echo PI; // Find the PI under the current namespace
echo \PI; // Force access to the global namespace

define() accepts the full name string

 define('App\PI', 3.14);

The effect is equivalent to const , but it is more flexible.

Summary: When to use which one?

Tips

• Use const first unless you need dynamic or conditional definitions.
• Use defined('CONST_NAME') to check if the constant exists.
• Constant names are usually capitalized to enhance readability.
• PHP 7.0 supports better expression support for const (such as const MAX = 2 * 1024; ).

Basically that's it. const is more modern and efficient, while define() is more flexible. Understanding their differences will allow you to write clearer and more reliable PHP code.

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