What are register_globals in PHP?

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
}