What are register_globals in PHP?

Can someone give some examples of what register_globals are?
And is global $user_id; considered a register global?

The register_globals directive:

register_globals is an internal PHP setting which registers the $_REQUEST array's elements as variables. If you submit a value in a form, via POST or GET, the value of that input will automatically be accessible via variable in the PHP script, named after the name of the input field.

In other words, if you submitted a form containing a username text field, the expression ($username === $_POST['username']) at the very beginning of the script would return true.

Its notoriety is attributed to the fact that it opens lots of security holes, especially for people that follow anything less than a strict coding style from a security perspective.

Classic example:

if(user_is_admin($user))
{
    $authorized = true;
}

if($authorized)
{
    // let them do anything they want
}

Now, if you visited that script in a web browser and the server had register_globals on, you could simply append ?authorized=1 to the URL and god-mode would be enabled!

The global keyword:

global is a keyword has little to do with register_globals.

Here is an example of its use:

$foo = 'bar';

baz();

function baz()
{
    echo $foo; // PHP warns you about trying to use an uninitialized variable
               // and nothing is output (because $foo doesn't exist here)
}

buzz();

function buzz()
{
    global $foo; // Enables the use of $foo in this scope

    echo $foo; // Prints 'bar' to screen
}

Everyone mentioning GET, POST, REQUEST, COOKIE has effect on register_globals=on.

I'm just writing this to let you know that -

$_SESSION will be affected aswell because of register_globals=on. http://php.net/manual/en/security.globals.php

That means - if you do as following -

$_SESSION[x] = 123;
$x = 'asd';
echo $_SESSION[x];

The output will be asd.

And this will cause serious security issues and bugs. I have experienced such a bad thing recently during using Hostgator shared hosting. By Default they have register_globals=on.

When you have register_globals=on, anything passed via GET or POST or COOKIE automatically appears to be global variable in code, this might have security consequences.

I.e. you click on url test.php?access_level=100 and you'll have $access_level = 100 in PHP.

When you do global $somevar - you are making your own global variable, which usually is not a big issue.

The register_globals setting controls how you access form, server, and environment. variables.

register_globals=On :

You can access form attribute without Global Arrays ( GET[], POST[] & REQUEST[] )

example: http://www.example.com/one.php?myinput=abc

You can access directly in one.php

echo $myinput; // abc

register_globals=Off :

You have to access all attributes only by Global Arrays.

example: http://www.example.com/one.php?myinput=abc

You have to access in one.php

echo $_GET['myinput']; //abc

As I understand it, if you have register globals turned ON, then anything passed in a GET or POST gets automatically translated into a variable in PHP.

for example:

http://www.domain.com/vars.php?myvar=123

without any further coding this would automatically get turned into a variable available to the rest of your php code

$myvar  //with a value of 123

With registered globals OFF, data passed in via GET or POST is NOT automatically translated into a variable, rather, you need to request it using the Superglobals $_GET, $_POST, and $_REQUEST, etc.

http://php.net/manual/en/security.globals.php provides some further information as to the security implications of this.

Others can feel free to correct me if I'm wrong.

edit:

in relation to your question re global $user_id;, this does not create a 'global' in the sense of 'register_globals'. It simply alters the scope of a variable within the PHP code.

For information re scope, see: http://php.net/manual/en/language.variables.scope.php

Register Globals :

> register_globals The feature causes data passed to a PHP script via cookies or GET and POST requests to be made available as global variables in the script.

Default Value : "0"

Changeable : PHP_INI_PERDIR

register_globals is affected by the variables_order directive.

NOTE:

This feature has been DEPRECATED as of PHP 5.3.0 and REMOVED as of PHP 5.4.0.

Global variables in php are variables that are always accessible. They are also known as superglobals. They are built in variables that are always available regardless of the scope.

There are nine superglobal variables in PHP. Some of these are relevant to this discussion.

1. $_REQUEST
2. $_POST
3. $_GET
4. $_COOKIE

Now, let's focus on the $_REQUEST superglobal. It is used to collect data after submitting an HTML form by user using the POST method.

$_POST and $_REQUEST could be used loosely interchangeably. But $_REQUEST also contains $_GET and $_COOKIE along with $_POST so you are never sure if your data came from a web form.

Now, as pointed out by @Tim register_globals is an internal PHP setting which registers the $_REQUEST array's elements as variables. It is also known as a flag in your php setting. It is typically set in the PHP configuration file known as php.ini file. This setting can have two values.

1. “on”
2. “off”.

An “on” value means that PHP will automatically create global variables for many server variables as well as query string parameters. This is not good and is a security risk.

register_globals is one of the parameters of php.ini file.The file was coming from "On" mode before PHP 5.3.8 version.If you change register_globals from Off to On, there would be some criticals about vulnerability of website.Register_globals's feature is that you can use variables without $_GET and $_POST variables.So, Any data which comes from form or URL line you can use the variable not need like $_GET or $_POST variables.Example we have this form :

<?php 
if(isset($_POST["myinput"])){
		echo $_POST["username"];
	}?>


<form action="" method="post">
	<input type="text" name="username">
	<input type="hidden" name="myinput">
	<input type="submit" value="Submit">
</form>

When you submitted the form you can see your username but when you change the situation of register_globals to "On" you have not written $_POST["username"]; you can access directly to the username variable by writing this code echo $username