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.

No, you cannot redefine a constant (except with runkit_constant_redefine),
that's why is called CONSTANT.

What you are looking for in the class is actually an object variable in array format:-

class user
{
  public $user = array();

  function load($user_id)
  {
    // etc
    $this->$user[$user_id] = something_else;
  }
}

In PHP 5.6+, you can import constants, under an alias. Such alias can overwrite an already existing constant.

The following snippet will print int(5):

define('ALLEGRO_ID', 3);
define('NEW_ALLEGRO_ID', 5);
use const NEW_ALLEGRO_ID as ALLEGRO_ID;
var_dump(ALLEGRO_ID);

Such mechanism could be legitimate in unit test, or other areas where magic behavior is tolerated, but isn't convenient for your use case, where a class to store your user settings would be more useful.

Finally, this behavior allows to create very confusing situations like use const true as false;.