PHP Constants: `const` vs `define()`

In PHP, there are two primary ways to declare constants: using the const keyword and the define() function. Each method has its own use cases and characteristics. This guide explains the crucial differences between these two methods.

Key Differences

1. Compile-time vs. Run-time

const: Constants declared with const are handled at compile time. This means that the value of the constant is determined and set when the script is being compiled, before it is executed.
```
const MAX_USERS = 100;
```
define(): Constants declared with define() are handled at run time. This means that the value of the constant is determined and set during the execution of the script.
```
define('MAX_USERS', 100);
```

2. Conditional Declaration

const: Because const is handled at compile time, you cannot conditionally define a constant using const. The const keyword must be used in a straightforward, unambiguous declaration.

    if($condition){
        const MAX_USERS = 100; // This will cause a syntax error
    }

define(): Since define() is a function and operates at run time, you can use it within conditional statements or loops.

    if($condition){
        define('MAX_USERS', 100); //This is perfectly valid
    }

3. Scope

const: Constants declared with const are always defined within the scope in which they are declared. When used inside a class, they are scoped to that class.

    class MyClass {
        const MAX_USERS = 100;
    }

    echo MyClass::MAX_USERS; //Outputs 100

define(): Constants declared with define() are always defined in the global scope, regardless of where they are called. This means they can be accessed globally.

class MyClass {
    function setConstant() {
        define('MAX_USERS', 100);
    }
}
$obj = new MyClass();
$obj->setConstant();
echo MAX_USERS;  // Outputs 100

4. Usage in Namespaces

const: When using const within a namespace, the constant is scoped to that namespace.

namespace MyNamespace;
const MAX_USERS = 100;
echo MAX_USERS;  // Outputs 100

define(): Constants declared with define() are always in the global namespace.

namespace MyNamespace;
define('MAX_USERS', 100);
echo \MAX_USERS;  // Outputs 100