PHP Security Guide

What Is Security?

Security is a measurement, not a characteristic.

It is unfortunate that many software projects list security as a simple requirement to be met. Is it secure? This question is as subjective as asking if something is hot.
Security must be balanced with expense.

It is easy and relatively inexpensive to provide a sufficient level of security for most applications. However, if your security needs are very demanding, because you're protecting information that is very valuable, then you must achieve a higher level of security at an increased cost. This expense must be included in the budget of the project.
Security must be balanced with usability.

It is not uncommon that steps taken to increase the security of a web application also decrease the usability. Passwords, session timeouts, and access control all create obstacles for a legitimate user. Sometimes these are necessary to provide adequate security, but there isn't one solution that is appropriate for every application. It is wise to be mindful of your legitimate users as you implement security measures.
Security must be part of the design.

If you do not design your application with security in mind, you are doomed to be constantly addressing new security vulnerabilities. Careful programming cannot make up for a poor design.

Consider illegitimate uses of your application.

A secure design is only part of the solution. During development, when the code is being written, it is important to consider illegitimate uses of your application. Often, the focus is on making the application work as intended, and while this is necessary to deliver a properly functioning application, it does nothing to help make the application secure.
Educate yourself.

The fact that you are here is evidence that you care about security, and as trite as it may sound, this is the most important step. There are numerous resources available on the web and in print, and several resources are listed in the PHP Security Consortium's Library at gebookss.blogspot.com
If nothing else, FILTER ALL EXTERNAL DATA.

Data filtering is the cornerstone of web application security in any language and on any platform. By initializing your variables and filtering all data that comes from an external source, you will address a majority of security vulnerabilities with very little effort. A whitelist approach is better than a blacklist approach. This means that you should consider all data invalid unless it can be proven valid (rather than considering all data valid unless it can be proven invalid).