Redefining Constants in PHP

Redefining constants in PHP

If you have the runkit extension installed, you can use runkit_constant_redefine():

runkit_constant_redefine("name", "value");

In most circumstances, however, it would be a better idea to re-evaluate why you're using constants and not something more fitting.

Is it possible to redefine PHP constants?

No, of course not. Then they wouldn't be "constants."

I can redefine constant in PHP without getting an error

To report all php error

error_reporting(E_ALL);

To report all php errors except notices

error_reporting(E_ALL & ~E_NOTICE);

And in your case, you are redefining a constant that has been already defined show PHP throws a notice. By using

error_reporting(E_ALL & ~E_WARNING & ~E_NOTICE & ~E_STRICT & ~E_DEPRECATED);

You are telling PHP not to report any kind of notices or warnings. You should remove them.

To get all warning and notices use

error_reporting(E_ALL);

Fore more information refer to the official doc, and have a look on examples.

Can you undefine or change a constant in PHP?

No. Constants are constant.

Reference: php.net/manual/language.constants.php

Change the value of a previously-defined constant

From http://php.net/manual/en/function.define.php (emphasis is mine):

define — Defines a named constant

From http://www.php.net/manual/en/language.constants.php:

As the name suggests, that value cannot change during the execution of the script

Redefining Codeigniter slug-based constants in the model per user

It not so much "unset" as "not yet defined". Each browser request to a URL (controller) is a unique process in the server. So the constant is not redefined but is newly instantiated for each URL request.

If you try to run $this->main_model->set_slug_based_constants($slug); a second time you will get a PHP error message stating that Constant CLOUDFRONT_DIRECTORY already defined.

Try this version of the controller to see the error.

class Content extends CI_Controller
{
    function __construct()
    {
        parent::__construct();
        $this->load->model('main_model');
        $this->main_model->set_slug_based_constants("Dave");
    }

    function index()
    {
        echo "The constant's value is <strong>".CLOUDFRONT_DIRECTORY."</strong><br>";
        $this->main_model->set_slug_based_constants("jruser");
        echo "The constant's value is  <strong>".CLOUDFRONT_DIRECTORY."</strong><br>";;
    }

}

It will produce output something like this.

The constant's value is Dave_cloudfront_directory

A PHP Error was encountered

Severity: Notice

Message: Constant CLOUDFRONT_DIRECTORY already defined

Filename: models/Main_model.php

Line Number: 13

The constant's value is Dave_cloudfront_directory

How to override a class constant in PHP?

If your users are extending your class, they can redefine the constant just fine:

class Foo extends Bar {
    const BAZ = 42;
}

It doesn't matter whether Bar already defines a BAZ constant or not. All you have to do in your code is make sure you're referring to the "current classes" constant using late static binding:

public function useConst() {
    echo static::BAZ;
}

Otherwise, you have the option of doing this:

if (!defined('BAZ')) {
    define('BAZ', 42);
}

class Bar {
    const BAZ = BAZ;
}

That's not really very elegant though. You should rather be using getters and setters on your class to allow configuration, rather than using constants for this purpose:

Bar::setBaz(42);

unset or change a defined constant PHP

When you use PHP's define() function, you're not defining a variable, you're defining a constant. Constants can't have their value modified once the script starts executing.

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