What security issues should I look out for in PHP

I just starting out learning PHP, I've been developing web apps in ASP.Net for a long time. I was wondering if there are any PHP specific security mistakes that I should be looking out for.

So, my question is what are the top security tips that every PHP developer should know?

Please keep it to one tip per answer so people can vote up/down effectively.

Solution 1:

(In no particular order)

1. Always check that register globals are OFF
2. Always check that magic quotes are OFF
3. Make sure you understand SQL injection attacks
4. Turn OFF error reporting in production

EDIT: For the "newbies" out there this is a basic why (and since I have time to explain this):

1. Register globals is an aberration. It's the ultimate security hole ever. For example, if register_globals is on, the url http://www.yourdomain.com/foo.php?isAdmin=1 will declare $isAdmin as a global variable with no code required. I don't know why this "feature" has made it's way to PHP, but the people behind this should have the following tattooed on their forehead: "I invented PHP Register Globals" so we can flee them like pest when we see them!

2. Magic quotes is another dumb idea that has made it's way to PHP. Basically, when ON PHP will escape quotes automatically (' become \' and " become \") to help with SQL injection attacks. The concept is not bad (help avoid injection attacks), but escaping all GET, POST and COOKIE values make your code so much complex (for example, have to unescape everytime when displaying and data). Plus if one day you switch this setting OFF without doing any change to your code, all your code and/or data is broken and (even more) vulnerable to injection attacks (yes even when ON you are vulnerable).

3. Your databse data is your most valuable thing on your site. You don't want people to mess with it, so protect yourself and read things about it and code with this in mind.

4. Again this can lead to security concerns. The error message can give hints to hackes on how your code works. Also these messages don't mean anything to your visitors, so why show them?

Solution 2:

Avoid using register_globals.

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