You want to access form input variables without allowing malicious users to set arbitrary global variables in your program.
Disable the register_globals configuration directive and access variables only from the $_GET, $_POST, and $_COOKIE arrays to make sure you know exactly where your variables are coming from.
To do this, make sure register_globals = Off appears in your php.ini file.
As of PHP 4.2, this is the default configuration.
When register_globals is set to on, external variables, including those from forms and cookies, are imported directly into the global namespace. This is a great convenience, but it can also open up some security holes if you're not very diligent about checking your variables and where they're defined. Why? Because there may be a variable you use internally that isn't supposed to be accessible from the outside but has its value rewritten without your knowledge.
Example 9-26 contains a simple example: imagine you have a page in which a user enters a username and password. If they are validated, you return her user identification number and use that numerical identifier to look up and print out her personal information.
Insecure register_globals code
Normally, $id is set only by your program and is a result of a verified database lookup. However, if someone alters the query string, and passes in a value for $id, you'll have problems. With register_globals enabled, your script could still execute the second database query and return results even after a bad username and password lookup. Without register_globals, $id remains unset because only $_REQUEST['id'] and $_GET['id'] are set.
Of course, there are other ways to solve this problem, even when using register_globals. You can restructure your code not to allow such a loophole. One way to do this is in Example 9-27.
Avoiding register_globals problems
In Example 9-27 $id has a value only when it's been explicitly set from a database call. Sometimes, however, it is difficult to do this because of how your program is laid out. Another solution is to manually unset( ) or initialize all variables at the top of your script. This removes the bad $id value before it gets a chance to affect your code. However, because PHP doesn't require variable initialization, it's possible to forget to do this in one place; a bug can then slip in without a warning from PHP.
9.15.4. See Also
Documentation on register_globals can be found at http://www.php.net/security.registerglobals.php.