PHP -What's the difference between global variables and constants

Question

According to many sources, register_globals (global variables that is) should be disables in your php.ini. Should I write define() in my code and use constants if global variables are disabled? Are those even related?

Answer 1

Constants, once defined, cannot be changed.

Don't use constants as variables. If you need to use variables within functions, pass them into the function itself. Use everything in the way it was intended to be used. Variables are variable and Constants are constant.

Answer 2

They are related in that they have global scope, but constants are meant to not change once defined, unlike global variables which the page can modify as it goes along. So just switching over to using define() instead of a global won't help much.

It's better if you refactor your methods to take the variables as parameters and rely on that to pass variables around.

Answer 3

A few things here.

First, the register_globals that you disable in your php.ini refers to an old PHP feature where any variable sent via a query string (GET) or form (GET/POST) would be converted into a global PHP variable. This is the functionality that is (and should be) disabled when you turn off register_globals. Even with this off, you can still define global variables in your application.

In general programming terms, global variables (not PHP's register_globals) are considered "bad" because when you encounter one as a programmer, you have no idea what other parts of the application might be using or changing it, or what effect your changes to that global might have. Also, if you're defining a new global variable, there's a chance you're going to overwriting an existing variable that someone else is relying on. When variables are defined locally (in a single function, or in other languages a single block) you can examine the local scope and usually determine what a change to that variable will do.

Constants, on the other hand, never change. You define them once, and they stay defined and no one can change them. That's why having global constants is considered "less bad" than having global variables.

Answer 4

Global variables aren't constant (you can change the value of a global variable, but you can only define a constant once).

Constants aren't always global (you can declare a constant in a class).

Also, global variables can be any type: scalar, array, or object. Constants can only be scalars.

I'm not going to say either constants or globals are good or bad. When used appropriately, they both have beneficial uses. There are security issues around the register_globals feature that are separate from more general use of globals.

Answer 5

Some constant examples:

<?php

// Certainly constant
define('MINUTES_PER_HOUR', 60);
define('DOZEN', 12);

// Constant, but specific to this application
define('GREETING', 'Dear %s');
define('TIMEOUT', 30);

// Configurable, but constant for this installation
define('DATABASE', 'mydb');
define('IMAGES_DIRECTORY', '/tmp/images');

// Not constant, or some other reason why can't be constant
$user = $_POST['userid'];
$days_of_week = array('Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su');

?>

Answer 6

Try this simple test:

fileA.php:

<?php
define('SOMEVAL', 2);
?>

fileB.php:

<?php
if(defined('SOMEVAL')) echo SOMEVAL;
else echo "SOMEVAL does not exists\n";
include 'fileA.php';
if(defined('SOMEVAL')) echo 'SOMEVAL='.SOMEVAL;
else echo "SOMEVAL does not exists\n";
?>

Then run fileB.php and you'll see that before you include fileA.php, SOMEVAL is not defined. So what this means is that before you define anything, it won't be visible to the script.

Answer 7

Something else to consider -- constants can't store things like arrays or objects, whereas something defined to $GLOBALS can be any variable type. So in some cases, if you need something to be global but it can't be stored to a constant by using define(), you might want to use $GLOBALS instead.

Also, register_globals and $GLOBALS are NOT the same thing!